I discovered this when I tried to purchase the 90s pack:
All music pack DLC that I have not purchased shows "0 songs" and "$0" (in the synthriders app) and won't let me purchase, with a 'something went wrong' error. The same oh-so-helpful error presents when trying to purchase the dlc directly from the store (tho price is listed there), regardless of being attempted in the headset, mobile app, or we page.
individual tracks that are part of the packs will allow me to purchase them.
Quest 3, and v77 recently installed, but I was on v76 when I first ran across it. I'm just bad about remember to report things in a timely fashion.