For anyone out there having trouble with this issue, I have run across (I think) the same thing a few times now, and the problem seems to be how the files are created. If you check the URL of the forms in question, they should have an extension of
'.xml', but forms created this way often show up as '.xsn'. The workaround is to replace the default '.xsn' form template with an empty '.xml' form, as documented
NOTE: You must disable all required fields to create the empty form, but when you re-upload the form with the required fields enabled, it will automatically remove your template. Thus, every time you change your form, you will need
to repeat this procedure (and save the new template to upload AFTER you have re-enabled your required fields).