It is a mistake to expect Plex to respond to inquiries like this. They only respond when the spirit moves them to do so. They said in roundtables last year that they were going to be more responsive to customers but that has not come to pass and I believe that it was said to make people wanting more and better communication to simply shut up and go away.
The "store progress" for audio files" feature is, like much else Plex has added is incomplete at best. Plex added that simply to, as the falsehoods in the roundtables did, shut people up.
There are many many unfinished or broken features that Plex has added just to get people requesting them to shut up and go away. The save position in an audio file was added to throw people wanting audio book support a bone. (or under the bus.) But it has never been thoroughly debugged or even finished.
Plex stopped development of many features Plex does not really want to add features that Plex believes are best for their users. Of course that really means features that are best for Plex.
I abandoned Plex for audio books some time ago and now all my audio is handled outside of Plex through a small computer running MediaMonkey. I found that was the only way I could have reliable audio playback and I suggest that everyone that really wants good playback and control of audio files find an alternative to Plex.
Plex uses new task/features as "avoidance therapy" so they do not have to keep working on the things that are not of current interest to them. Plex has a short attention span disorder that prevents them from finishing anything or, maybe, Plex should just be called quitters.