Burmese is tonal, and has both the noun "Myanma" and the related but not identical adjective "Myanma". The noun has a low tone and a long "a", which reflecting British pronunciation was spelled "Myanmar". The adjectival form uses the creaky tone with a short "a", hence it's supposed to be spelled "Myanma" without the r.

In practice, though, nobody cares and everybody says "Myanmar" for both, even the official English translation of the constitution, which says "Myanmar language".

"Myanman" or "Myanmarese" are both wrong though.