Document has been modified or damaged after signature

Hi, we are using a remote webservice to digitally sign some PDF documents we generate. They actually get signed twice, the first time with our own certificate (will be a QESeal in production). The second time they will be signed by our client using their own european qualified signature. Both our software and the clients' one use the iText library to do the signing (both v7.1.18).

While testing this flow we discovered that Adobe Reader v2023.001.20093 marks our signature as invalid when the client applies a visible signature. However a binary diff of the files shows that the additional signature is appended and doesn't modify at all the initial file (see the attached screenshot).

This does not happen if the client signs with an invisible signature.

Why does Acrobat shows that the document has been modified when applying a visible signature?

Thanks in advance,

Luca