During Runtime, when clicking a button in Nintex Form for Office 365, the button that has been configured with a 'Confirmation message' which contains an apostrophe " ' " character, will not return a pop-up dialog as expected. 
The Form will be closed immediately without a confirmation message. 

Modify 'Confirmation message' for the button control, and for any apostrophes or any other special character add slash '' before the character. 

For example:

Incorrect value: You've submitted the form. 
Correct value: You've submitted the form. 

During the runtime, you will receive this value: You've submitted the form.