"Different user's signature" message for my own signature

Summary

I've configured SSH key signing of my own commits, but when viewing the commits on GitLab it says it's a different user's signature. If this is actually the case, and not just a bug in GitLab, then as far as I understand it would mean that my SSH key has been compromised.

Steps to reproduce

1. Create an SSH key pair in ~/.ssh

2. Log into gitlab.com

3. Add the public key to https://gitlab.com/-/profile/keys

4. Add the following to ~/.gitconfig:

   [commit]
   	gpgsign = true
   
   [gpg]
   	format = ssh
   
   [gpg "ssh"]
   	allowedSignersFile = /home/username/.ssh/allowed_signers
   
   [user]
   	signingkey = /home/username/.ssh/id_rsa.pub

5. Add the following to ~/.ssh/allowed_signers, replacing the email address and public key with your own values:

   username@example.org namespaces="git" [the contents of ~/.ssh/id_rsa.pub without the square brackets]

6. Commit something with the git commit -S flag to sign the commit

7. Verify that git show --show-signature says 'Good "git" signature for username@example.org with RSA key SHA256:[omitted]'

8. Push the commit to GitLab

Example Project

engmark/root!383 (commits)

What is the current bug behavior?

The commit is marked as "Unverified" in GitLab, with a message like this one:

Different user's signature

This commit was signed with a different user's verified signature. SSH key fingerprint: [omitted]

What is the expected correct behavior?

The commit should say it's verified.

Relevant logs and/or screenshots

Other info

My "commit email" on GitLab is different from the email address which is in my actual commits..

It might be relevant that I have configured two SSH keys in GitLab.

I always type my email address in lower case, so this isn't relevant.

Output of checks

This bug happens on GitLab.com