Acrobat Pro 2017 Digital signature disappears when file saved

One of my users is having an issue with Acrobat Pro 2017.

He generally has 20-50 documents per day that need to be signed, so of course he uses a digital signature in Acrobat (Fill & Sign).

Most of the time, he selects the Fill & Sign option, drags his saved signature to the correct location, and adds a date.

Then, when he goes to 'Save As', and selects the location to save the document, as soon as he clicks the Save button, the signature and added date disappear (although the file does in fact save).

Immediately, he does the same process again without closing the file, this time overwriting the previously saved file, and the signature/date will stick.

Move on to the next file, same problem.

This effectively doubles up the time and effort needed to sign these files.

It does not matter if the save destination is a local vs network path, whether the destination file already exists or not does not seem to matter (initially overwriting the original source vs saving to a new location).

Even a brand new blank PDF with nothing but the signature added will have this issue.

Every once in a while, it will actually work without having to duplicate the process, but it seems to be random, and quite infrequent...

Any suggestions would be appreciated.

Thanks