This seems to be a long running issue as I've found roundabout solutions in forums and mention of this on github. Even in Moodle 4.3, we are encountering this bug of allowing question bank categories to parent itself causing it to disappear on the front-end. Through front-end logs, we can see a looping error.

To reproduce:

• Go into any question bank category and click the cog to edit
• Change the name but keep the same parent, and save
• This causes the category to completely disappear

When searching the db, we found it was because of the parenting of itself that caused this looping issue.