Merge request Approval - After add user to the list, some users not showing up in the merge request to approve

Summary

A few days ago we updated from 11.3 version to 11.8.1. At that moment we discovered some users in the approval list of several projects where missing once you want to approve the merge request, but they exist in the project configuration. If you remove them from the list and add again happens the same, but not for all the users, just random ones, and not always the same ones between projects even being part of the same parent group of users.

Steps to reproduce

• Add a list of users to approval list in a project.
• Create a merge request.
• Some of the users will be missing.

What is the current bug behavior?

Some users are missing at time to approve a merge request, but in the project config they are in the list.

What is the expected correct behavior?

You should see all the users specified in the merge approval list once you want to approve a MR