Why DOM explorer shows value=\on\" for all the MultiChoice options"

I'll be honest, I have no idea why your choice field is setup that way.

I can tell you that I'm running SharePoint 2013 on-prem with Nintex Forms Enterprise version 2.11.4.30.

Here's what mine looks like when I setup the exact same field in a form:

Field Settings

Form Layout

Options HTML

You can see that each of my <input> elements have their value attribute set to the text of the adjacent <label>, and the parent <span> element.

Here's the complete HTML code for my entire 'MyCheckboxes' control:

<div 
     data-uniqueid="c0a89c70-0781-4bd4-8623-f73675005e02" 
     data-controlname="MyCheckboxes" 
     class="nf-form-input nf-section nf-filler-control nf-filler-highlightonvalidationerror  nf-resizable nf-resizable" 
     formcontrolid="c4368cc1-3503-423a-b710-dbf83b0d4f34" 
     data-multiselectcontrol="True" 
     style="overflow:visible;height:120px;width:500px;top:115px;left:200px;z-index:100;background-color:;" 
     data-controlid="c4368cc1-3503-423a-b710-dbf83b0d4f34" 
     data-formcontroltypeid="c0a89c70-0781-4bd4-8623-f73675005e02" 
     data-enabled="True"
>
     <div class="nf-filler-control-border">
          <div class="nf-filler-control-inner">
               <div>
                    <table 
                         id="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba" 
                         class=" nf-choice-checkbox nf-associated-control" data-use-attribute-as-value="data-nfChoiceValue" 
                         style="width:100%;"
                    >
                         <tbody>
                              <tr>
                                   <td>
                                        <span data-nfchoicevalue="Not Applicable">
                                             <input 
                                                  id="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_0" 
                                                  type="checkbox" 
                                                  name="ctl00$ctl45$g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef$ctl00$ListForm2$formFiller$FormView$ctl22$194d546f_2e47_4ad9_9c6e_d49a45ae14ba$0" 
                                                  value="Not Applicable" 
                                                  formcontrolid="c4368cc1-3503-423a-b710-dbf83b0d4f34"
                                             >
                                             <label for="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_0">
                                                  Not Applicable
                                             </label>
                                        </span>
                                   </td>
                              </tr>
                              <tr>
                                   <td>
                                        <span data-nfchoicevalue="Option 1">
                                             <input 
                                                  id="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_1" 
                                                  type="checkbox" 
                                                  name="ctl00$ctl45$g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef$ctl00$ListForm2$formFiller$FormView$ctl22$194d546f_2e47_4ad9_9c6e_d49a45ae14ba$1" 
                                                  value="Option 1" 
                                                  formcontrolid="c4368cc1-3503-423a-b710-dbf83b0d4f34"
                                             >
                                             <label for="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_1">
                                                  Option 1
                                             </label>
                                        </span>
                                   </td>
                              </tr>
                              <tr>
                                   <td>
                                        <span data-nfchoicevalue="Option 2">
                                             <input 
                                                  id="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_2" 
                                                  type="checkbox" 
                                                  name="ctl00$ctl45$g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef$ctl00$ListForm2$formFiller$FormView$ctl22$194d546f_2e47_4ad9_9c6e_d49a45ae14ba$2" 
                                                  value="Option 2" 
                                                  formcontrolid="c4368cc1-3503-423a-b710-dbf83b0d4f34"
                                             >
                                                  <label for="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_2">
                                                       Option 2
                                                  </label>
                                             </span>
                                        </td>
                              </tr>
                              <tr>
                                   <td>
                                        <span data-nfchoicevalue="Option 3">
                                             <input 
                                                  id="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_3" 
                                                  type="checkbox" 
                                                  name="ctl00$ctl45$g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef$ctl00$ListForm2$formFiller$FormView$ctl22$194d546f_2e47_4ad9_9c6e_d49a45ae14ba$3" 
                                                  value="Option 3" 
                                                  formcontrolid="c4368cc1-3503-423a-b710-dbf83b0d4f34"
                                             >
                                             <label for="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_194d546f_2e47_4ad9_9c6e_d49a45ae14ba_3">
                                                  Option 3
                                             </label>
                                        </span>
                                   </td>
                              </tr>
                         </tbody>
                    </table>
               </div>
               <span 
                    id="ctl00_ctl45_g_79a9bcbb_ba3e_411a_8651_7926bae0c2ef_ctl00_ListForm2_formFiller_FormView_ctl22_ctl03" 
                    class="nf-boundvalidator nf-validator-error" 
                    style="display:none;"
               >
               </span>
          </div>
     </div>
</div>
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Maybe you can use it to compare with your code.