CI is only run on a subset of 4 branch after pushing changes in more than 4 branches

Summary

In a git repository with multiple branches, CI will only queue and run a subset of 4 branches randomly (all other directly went to unknown) if you push more than 4 branches. I asked in IRC and it has been suggested to file a bug report. If this is some configuration problem, I would like to know what is to change to queue all CI pipelines. The problem also exists on GitLab.com, so I do not think that this is a configuration problem on my private GitLab FOSS/CE instance.

Steps to reproduce

I created a test project that shows the behaviour.

What is the current bug behavior?

It only queues and runs 4 CI pipelines as a random subset of all pushed branches - all other branches with changes go directly to unknown. Also, on the individual commit on an unknown branch, there is no CI-icon next to the commit id, which should be the case

What is the expected correct behavior?

All branches should be queued and not directly go to unknown. I can work around by executing the pipeline manually or let it be executed via a timed pipeline run, but the whole idea behind CI is to run it on changes and this is impossible right now.

Relevant logs and/or screenshots

The current test in my repository show this in the README.md:

Output of checks

This bug happens on GitLab.com and in my private repository.