Invalid signature

You mention that this happens in one document, not in all documents; thus, the problem is related to this very document and we'd need it for analysis. Thus, if possible, please share it.

In general one possible reason for signing breaking signatures is that the earlier signatures forbid any changes, even additional signatures. But in such a case Adobe Acrobat usually does not allow adding another signature. Thus, this most likely is not the issue in your case.

Another possible reason are certain errors in the original PDF. Adobe Acrobat generally is very lax when displaying or handling slightly broken PDFs (which has led to a considerable amount of such broken PDFs in general use). But in one situation it is very sensitive to some such errors, and that is when validating a signed PDF with changes after signing (e.g. with another signature added); then those errors can result in negative validations.

Thus, I'd assume that the document in question has such a normally ignored error.