Text channels missing for user who accidentally clicked "something"

I'm the creator of a server. I have three categories, each one with multiple text channels.

A couple of hours ago, one of the members was in one of the categories, in a text channel, and with his cursor in the menu of channels on the left hand side, was jostled and accidentally clicked something. Now, channels only in that category disappear when he enters one and then leaves to go to another one.

I have viewed the server as all his roles, one at a time, or more than one at a time. Everything looks fine when I view it. He is having the same problem both on his laptop (Windows) and mobile, so I know it's not a matter of his OS being outdated or something like that. No other user is having this issue.

I did see he accidentally created an invite to one of the text channels, a few minutes before he started having issues and contacted me. Deleting the invite did not fix the problem. Checking that permissions were synced in the category did not fix the problem

I had him leave the server, and then I invited him back. For one minute, it all was back to normal, and then as soon as he entered a text channel, in that one category, the issue came back.

His discord is up to date. Does anyone have ANY idea what the issue is???