This is a known limitation in how we link commit hashes in descriptions. The site will turn anything that looks like a commit hash into a link, even if there's no commit by that hash in the repository. This is done for performance reasons.
I've put an issue in our internal backlog to revisit this behavior and look into making it only link valid commits. You can follow this issue for further updates.
Would be great if a commit hash would always be linked to the actual commit. I do not know about the likelihood of hash collisions, but if that's not too high, this would be great. If there is a collision, the commit in the same repository should of course be preferred. This would also solve the feature request in .