job stuck: protected branch-only runner; branch protected after commit

Summary

a (shared) runner which only runs on protected branches isn't picking up a job from a recently-protected branch

Steps to reproduce

• create an (unprotected branch) with a .gitlab-ci.yml that needs to run something on a shared runner which only runs for protected branches (was a manual job in my case; but I don't think that'd change things)
• push the code
• notice the job being stuck (as the branch is not protected yet; and the tags limit to that runner
• protect the branch

Example Project

Bug requires setting up a protected (globally) shared runner, so N/A

What is the current bug behavior?

The job remains stuck

What is the expected correct behavior?

The job getting picked up by the runner

Relevant logs and/or screenshots

p.s.: Confirming that the same runner (with the same tagset) does work on other (long-time)protected branches like master
p.s.2: job got picked up by that runner as soon as I disabled protected-branches only; even though the branch is protected