Intermittent error pushing merge commits from upstream committers without CLA

Summary

While trying to merge MR eclipse/oniro-core/meta-clang!4 (merged) (which is a merge request with an upstream repository run by people without CLA), hitting the "Merge" button first showed the "It's all going to be great" message, then switched to a message pointing to https://api.eclipse.org/git/eca/status/114a301baa61370286ee656083886123/ui (which, at that time, listed several errors for some of the commits -- Author doesn't have a CLA/Committer doesn't have a CLA or both). That message eventually went away and was replaced with the Merge button again. The merge was silently ignored. Trying again resulted in the same. Strangely enough, trying a third time (not for the sake of trying again, but for the sake of getting the exact api.eclipse.org URL again) resulted in a successful merge

Steps to reproduce

1. Create a repository in the eclipse namespace that forks an external upstream repository with committers/authors who don't have CLAs in place
2. Fork the repository to a personal namespace [this might or might not be necessary to reproduce]
3. git merge an upstream branch that has new commits from people without CLA
4. git push the branch to the personal repository
5. Start an MR to the eclipse repository
6. Try to merge it through the gitlab web UI

What is the current bug behavior?

Fails (apparently at random) pointing out missing CLAs

What is the expected correct behavior?

MR is merged, commits appear in repository

Relevant logs and/or screenshots

Unfortunately the successful merge apparently overwrote https://api.eclipse.org/git/eca/status/114a301baa61370286ee656083886123/ui - but there may be more meaningful logs on that machine that I don't have access to.