Unable to determine how a document was locked

We are running into issues here and there where our users report a signed PDF document being locked for editing, leading to nobody else being able to add signatures or make further adjustments to the document fields.

I have been unable to replicate the issue on my end, since I am only getting the option to "lock document after signing" after signing the LAST signature field on the document. The example this user has provided to me is an example where only ONE of the multiple signature fields are signed, yet somehow the document is still locked.

Is there an easy explanation as to how someone is able to lock an Adobe pdf document after signing only ONE of the signature fields? I am unable to determine the method in which this document was locked.