The thing is - interfaith conversions happens when owner is in same religious group (at least from what is known to me on those events) - and Beja is Muslim. I suppose these have been converted back when territory was under Ethiopia, but three provinces? What is MTTH of that? Need to recheck - maybe Ethiopia completed Innovation...Random event, that changes religion in province. Seems to be more frequent this patch. Also works for other heretics, like Orthodox. I don't know, if this event worked for heretics in previous patches. I'm running Orthodox Swarm Austria right now and every couple of months there's at least 1 province, that randomly flips Protestant or Reformed. I think, that I even had 1 province flip Sunni in Russian region.
Not being picky, but looks like it was vice versa - Manchu claimed Mandate and then formed Qing. One cannot form Qing, unless they are SoH or empire - and the second is fairly impossible looking at the size of country. And yeah, I'm fun at parties...First time I have ever seen Qing form, and then claim the mandate as well.