GitHub Merge failure with existing GPG keys

Question

We have multiple github repositories, which require signed commits for pull requests. Sometimes, we are not able to merge pull requests due to the warning "Commits need to be signed", even though the commits are signed. It looks like the person, who created the pull request can merge it (Sqash and merge, even if it only consists of 1 commit), it happened with both accounts being the initiator of a pull request, so it seems neither to be a problem with one specific GPG key nor with the setup of the Git Clients. Any ideas or questions to go further in resolving this?