I also have this problem.

I think that the problem is a bad generated .p12 file.

Try again generating the file with openssl:

openssl genrsa -out private.pem 2048
openssl req -new -sha256 -key private.pem -out csr.csr
openssl req -x509 -sha256 -days 365 -key private.pem -in csr.csr -out certificate.pem
openssl pkcs12 -export -out identity.p12 -inkey private.pem -in certificate.pem

Then use the file "identity.p12" for sign the PDF.

I believe that the problem is not the certificate, because I use a certificate generated by an official issuer and this problem still happens, and I already used this certificate and it worked, but from some moment until now this problem has been happening.

I had a similar issue which could be solved with these changes: #61.

My certificate was also generated by an official issuer which came with a root and an intermediate certificate. Unfortunately, the intermediate certificate is currently not supported by this library (which will be changed by the PR #61). This results in not trusting the signed pdf because the trust chain can only be made via the intermediate certificate.