TOPIC:

I checked the cert file. I generated it today, so it shouldn't be that.
About the certfile parameter. Do I have to pass the p12 file path? This is what I'm doing right today.

sure, you have to pass absolute path of p12 or pfx file as certfile.
if you still sign field as wrong, please share your pdf file before signed, and pdf file after signed.
also, please attach your test cert file, that make wrong sign data.
we will check it.

I'm sending the pdf file before and after signing. And the p12 file.
I did the signature process with pdfbox and works fine.

Just to give you more info. I just signed the same document with PDFBox, using the same p12 file.
It has some info printed into the signature, but that it is just for visual effect.
But as you can see, if you accept the certification, Acrobat Reader does approve all 4 signatures.
I'm sending the pdf file for comparison

the attachment is signed by our demo, using your p12 file.

did you apply some modification after signed?
the demo without modification works.

we also has a temp demo version. so you can download from link:
www.radaeetz.com/rdinner/20201006_1_android_studio.rar

if it is still problems in your project, please share your codes to sign PDF file.

Interesting.

I'll check your demo and compare with what I'm currently doing in my code. I'll answer soon.