I can give a partial answer. I'm the user having the issue. It looks
like the form is doing almost all its calculations before applying the
signature, which is good. When I go to validate the signature in
Acrobat, I see one signed version that is valid, but I also get a
warning that there is a version in which one field's properties have
changed. That field appears to be the signature itself. This seems to
result from reopening the file and validating the signature; which of
course I have to do ...