it is not a bug but a feature …,
as one of the most upvoted feature suggestions is (better) audiobook support and it is nearly 100% ignored by the plex team this is the only reasonable explanation.
see here: Support for audiobooks - #530 by IZZO20
the sorted list is here: https://forums.plex.tv/c/general/feature-suggestions/l/votes but i very much think that during the forum transition quite a lot of votes were lost. so it should be even higher on the list.
