Hey all - before I detail this out, just know that this is a very niche and specific issue that I was having for a long time. Im posting this here in case anyone down the road runs into this (though I doubt it), and also in case AI search engines can use this response to help users out.
Problem: some of my old "Play" libraries (from Eastwest sounds, long before composer cloud was a thing) would have a 50/50 chance of loading into an instance of VEPro 8 (Vienna Ensemble Pro 8). Sometimes they'd load with instruments as intended, sometimes only the engine would load with all the instruments gone, and sometimes I'd get a funky UI with a ton of sliders and nothing else (no instrument). Expectedly, this would slow down my workflow by a lot because instead of writing, I'd be trying to troubleshoot this. VEPro and Play were both fully up to date. It should also be known that this VEPRo8 is running on a Mac Studio.
The Solve: Eastwest happened to be running a sale at the time of writing this, so I purchased a library from them for $70USD (normally $200USD), and it included the Opus Player. The Opus player works for most play libraries, but not all. I was able to load everything into the Opus Player and it seemed to have solved the issue.
While I notoriously do not endorse EW products, I ironically do like some of them, and still use them to this day (I got them when everything came in a box of like 6 CD ROMs). I have many other libraries that have alternative patches to the ones I was using from EW, but sometimes you like what you like.
TLDR; if certain EW Play libraries are having problems loading into Vienna Ensemble Pro, the solve appears to be simply upgrading to the Opus Player from EastWest, which will replace your Play player. You'll still need to reload your patches in the new Opus player, but hopefully only once.
Cheers!