Wrong error message "Source branch does not exist. Please restore it or use a different source branch"

This appears to be the same problem as:

https://gitlab.com/gitlab-com/support-forum/issues/1069 https://gitlab.com/gitlab-com/support-forum/issues/4559

Since we were referenced to open new issues in #4559 I'm doing so. Hope it's not just noise.

I've seen this issue with the following two MRs (private repo):

https://gitlab.com/cynerio/onprem/merge_requests/456 https://gitlab.com/cynerio/onprem/merge_requests/459

In both cases since I could not wait, I closed them, merged manually and pushed to master.