Signatures not properly flattened with PdfMerger

[brz]

I am trying to merge multiple PDF files using the PdfMerger class. The clickable box to verify the signature is still present in the resulting pdf: When clicking the box Adobe complains there's an error with the signature: (which translates to: "Error verifying signature. The signature contains incorrect, unrecognizable or corrupted data. SigDict /Contents illegal data")

Ideally, the visual representation of the signature should still be present in the PDF, but it should not be a clickable formfield anymore.

Also, any unsigned formfields are still present in the resulting PDF:

[EliotJones]

Potentially related to https://community.adobe.com/t5/acrobat-discussions/another-disappearing-signature-situation/td-p/13531866