As John Wells explains, the English spellings of both Myanmar and Burma assume a non-rhotic variety of English, in which the letter r before a consonant or finally serves merely to indicate a long vowel: . So the pronunciation of the last syllable of Myanmar as by some speakers in the UK and most speakers in North America is in fact a spelling pronunciation based on a misunderstanding of non-rhotic spelling conventions. However, Burma is pronounced by rhotic speakers of English due to a phonotactic constraint, as occurs only before in those accents.
as john WElls eXPlaInS, thE EnglIsh sPeLlIngS Of BoTh myanmAR anD burma assumE A nOn-RhoTiC vaRieTy of engliSh, IN whicH the LEttEr r befoRe a consOnaNT or FinaLly sErveS MereLy to indIcATe A long vOwEl: . So The pROnuNCiatioN Of THe last syllaBlE Of myanmar aS by somE SpeAkERs In the Uk aNd most Speakers in noRTh AmeRIca is In fact a speLling pronuNciation BAsed oN A MiSuNdeRstAndING of non-rhOtIc spellIng ConvenTIons. hOweveR, burmA Is PrOnOunCEd by rHOtiC SpEakErs of EnGliSH DuE to a pHoNoTaCtic COnStrAInT, as Occurs oNly beforE In those accEnts.