I’m not sure. For the record, it could just be a bug or an unexpected edge case, not necessarily a specific government intervention, but regardless it does seem more fragile than it should be considering Signal’s ethos.

For the military to have done that on purpose they would have to have had external help with planing. It's to high level.

So I'd assume accidentally with something about the cell phones changing making Signal and Telegraph want to re-authenticate.

Or the person has cleared their phone for obvious reasons which also logged them out.

Not sure if this is relevant but the Tatmadaw are majority owners of Mytel, which has one of the largest networks in subscriber base and hardware infrastructure. They never really gave up control of Myanmar in a strategic sense.