American young person here: I do the glottal stop thing with "button" but a flapped r with "butter." I think it has to do with the "n" - I would also do the glo

American young person here: I do the glottal stop thing with "button" but a flapped r with "butter." I think it has to do with the "n" - I would also do the glottal stop for "bitten" but not for "bitter." For me the second syllable is a pure syllabic /n/ - no preceding vowel or consonant.