Not that I think Plex is perfect, but I have to add my support here for the way it operates in this regard. A few things:-
Computer filesystems work more efficiently with structured storage and fewer numbers of files in any single folder. Simply throwing more than a thousand files into a single folder is just not good practice. Daft in fact. If nothing else, forces you to modify file names just to avoid duplications and makes maintenance a nightmare.
Plex works in a certain way with regard to cataloguing media files and associated metadata and if you follow its rules, it works. I am normally averse to software that insists I abide by its rules rather than it being able to work with how I want to do it, but in this case, Plex has got it right. Music media storage SHOULD be structured in this way, not just for Plex’s benefit. I cannot imagine anything worse than struggling with over a thousand media files in a single folder/directory. Madness.
I have over 25,000 music files and I have repeatedly created Plex libraries that now faultlessly import all those files, read their metadata and associated artwork files and add other interesting data from Internet sources. As I said, repeatedly. In an effort to achieve exactly what I want, I made some adjustments to the metadata to establish what works for me in both iTunes and Plex.
In my experience it is incorrect to state simply throwing thousands of music files into a single folder is some sort of industry standard. It is no such thing. Some software may deal with it as you want, but when I have tried this on a small scale, the player (most recently a Sony in car head unit) just plays each file in some order it decides and makes NO attempt whatsoever to read the metadata and present a structured view to the user. Give me Plex in the car.
So, don’t try and throw lots of files in a single folder and then complain that doesn’t work well in Plex. Just DON’T DO IT. It has been repeatedly stated here, even in this very thread that it’s a bad idea and is likely to not work. So why keep insisting on doing it. Who was it said the definition of insanity was to keep doing the same thing and expecting a different result. Plex has changed and it no longer does what you want, but that wasn’t good practice anyway (for multiple reasons as explained). This is NOT A BUG. Plex now places greater importance on music files’ storage structure, but this is what you should be doing anyway and there are multiple programs that can easily create this structure for you so it’s not even hard to set it up in the first place.
In this instance, Plex does a great job. The problem is not with Plex.
Less politely, stop whining about how Plex doesn’t do what you want. You have been repeatedly told what is required in order to achieve what you want and in any case, that is the best way to do it. If you refuse to comply, it ain’t gonna work for you. Simple as that. So JFDI.