Cannot DecryptAndVerify file, but can Decrypt

[poiug07]

I try to decrypt and verify the file given to me by an external party(most likely using gpg), but DecryptAndVerify fails due to verification exception. However, I can successfully Decrypt and get the contents of the file. I suspect the issue is the following:

• I have a public key with 2 subkeys (as viewed in Kleopatra).
• gpg --list-packets looks like this, schematically as I don't have keys/files now and cannot provide them anyway:

[0] their_key
[1] my_key
[2] data
[3] signature

• I debugged into the following code in the library that causes Verification to fail:

_verificationKeys = new Lazy<IEnumerable<PgpPublicKey>>(() => publicKeyRings.Select(Utilities.FindBestVerificationKey).ToArray());

_verificationKeys contains only one subkey of two (that is marked as primary key in Kleopatra), the other one is missing. When message is viewed with --list-packets, it shows keyId of that missing subkey.

I am a complete noob in PGP and cannot generate minimal sample with given properties myself. I am really sorry, that I cannot provide more context.