I can reproduce this in the browser (https://gitlab.com/api/v4/users?search="MYEMAIL@ADDRESS" returns an empty array, whereas https://gitlab.com/api/v4/users?search="MYNAME").
The right place to report bugs is the GitLab issues list. I couldn’t find anything relevant with a quick search but you might do a bit better than me!
There should be a bug template if you need to raise a new issue, otherwise do upvote and comment on any relevant issue you find, and that will make it more likely to be fixed.