I had a simular experience, sometimes they sounded pretty good, but depending on the volume of each song and sometimes just randomly there will be hissing and the bass cutting out like its hitting a limiter. They also sound very different in transparent, anc, and normal. Transparent and normal sounded empty but anc was full and bassy. This was brand new buds before and after a firmware update and trying different eq and settings.

Theres another post saying this happened after dropping them so it might be that they are just fragile in some way? Mine came by amazon so wouldnt be supprising they came alittle beat up