I regularly visit the Mathematics Educators chatrooms and I used to see three. But starting last March 16, 2017, the Mathematics Educators Meta chatroom disappeared from the list. (Clicking the "Show frozen/deleted rooms" button doesn't reveal it.) The room is still active and posts there can be seen and made.
Why did this room disappear from the list?
Sorry about that; this was fallout from the https switchover; the chat rooms hadn't had their individual hosts updated to reflect the "meta.*.stackexchange.com" => "*.meta.stackexchange.com" switch - so it was applying the wrong filter. I've now fixed this. Thanks for reporting it.
Perhaps somebody will be able to expand on this, but at least I can tell you how to find the "missing" chat room.
If I display rooms associated with Math Educators, currently I see two rooms. And I see one additional room associated with Meta Math Educators. I can only get the later link by manually changing url. (If I click on list of sites in chat, I am not able to find Meta Math Educators.) This room is the only room displayed in the latter list and the first two rooms are this one and this one.
If you have all three rooms among your favorite rooms (as I do) and choose to display favorite rooms, you should see all three (alongside other rooms you favorited).
The rooms for main and meta used to be displayed separately. This changed in March 2016, see this answer.
So my guess this is a bug related to recent changes of urls for per site metas. (You probably noticed that the url for meta changed from meta.matheducators.stackexchange.com to matheducators.meta.stackexchange.com.)
If my guess is correct, this should probably be reported as a bug on the main meta. (Although having such "hidden" room might have some advantages.)
