First of all, thanks for the reply!
I tested this with metaflac and I know that beets reads the TRACK tag, but since the tag is in the format “/”, it only reads both of there correctly for mp3 files and not for flac files.
I have noticed, that the tracktotal and disctotal are normally seperate tags in FLAC Vorbis Comments, but this would make managing my music files way easier, but maybe it would be better to change how my tagger saves those tags, if that’s possible.
I’ll try looking into it in the next days, but if anyone has any more suggestions, I’d appreciate that.