[x] Your issue is based on the latest commit
[x] State your OS and OS version: MacOS 13.6.3

Here is my information:

$ pdfcpu version
pdfcpu: v0.7.0 dev

$ pdfcpu validate pp.pdf
validating(mode=relaxed) pp.pdf ...
validation error (obj#:13): dict=formFieldDict required entry=Subtype missing

$ pdfcpu merge invoice.pdf ba.pdf pp.pdf
writing invoice.pdf...
ba.pdf
pp.pdf
dict=formFieldDict required entry=Subtype missing

If you need the file (the file contains the PII), I will email you.

Thank you for your help!