We have Classic 365 form, with a rule checking for isNullorEmpty on a Pulldown, disabling the Submit button if it is empty. And this has worked as far as I could tell.
Now I see it has not worked in all cases.
This pulldown, pulling filter values from a separate list, does not have a default value set, and is not saved to list column. It is only used as a filter on that separate SharePoint list. When a selection is made in the pulldown, the form pulls in filtered data from that list, into 2 Calculated Value controls, which are saved out into SharePoint columns in the current list (to be used by an approval workflow after Submit is pressed).
Only after a selection is made in the pulldown, should the Submit button become active.
But we have found over 20% of the forms have saved without the list pulled-in from the other list. I can only imagine that the pulldown has is holding a previous selection in cache. I have not been able to reproduce the problem, so I am at a loss as to what has caused it.
As a precaution I have added an extra rule looking at one of the Calculated Value controls (which should always have data), so that the form cannot be submitted if this is empty. This should prevent reoccurrence. But I would really like to understand the cause (as would my company-internal customers).