When Combining PDFs Digital Signatures Disappear

I agree. The only surprise is if it seems to remain. It suggests your staff aren't properly validating signatures, maybe they are only looking on the pages (a common habit, but a dangerous one).

try67 is right of course when clarifying that combining multiple files (unless doing so in a portable collection, aka portfolio) will break any digital signature in those files.

Thus, it is not surprising that Acrobat does not include the signatures in the combined result.

It is surprising, though, that it does so only on "some of the documents being combined". Can you determine for which kinds of documents the signature remains?

It's not an issue, it's by design. A signed file can't be edited, as that invalidates the signature. You need to first combine the files, and then sign them all as a single document, or you can combine the signed files as a part of a Portfolio.

PS. If you print a signed file the digital signature on it becomes meaningless, as it can't be validated.