After some experimentation, I've found that the problem seems to occur if there are subforms on a master page. Once I removed the offending subforms from my form, the signature validated correctly.

I'm having the same issue as well. In addition to the additions/deletions to the master pages, the signature panel indicates that a field on the title page of my form was filled in after validation. I ran a temporary script on the signature field's postSign event, using Acrobat's signatureGetModific...

getField's argument has to be the complete path to the signature field, including explicit instance indices, e.g. "form1[0].page1[0].Main[0].DeptHeadApproval[0]".