This has been bothering me for a long time; even with no music on my iPad and the app itself offloaded, there was 2.75 GB of storage being used up by the Music app (per Settings > General > iPad Storage). I found a solution, using iMazing as suggested above. Here's what I did:
-- Remove all music from iPad
-- Offload the Music app itself (you cannot delete it)
-- Plug in iPad, let it backup and sync
-- Run iMazing
-- in iMazing, go to iPad > File System
-- Go to Media/iTunes_Control/iTunes/Artwork/
-- In there are two folders, Caches/ and Originals/.
Both of therm have hundreds/thousands of files.
-- Delete contents of both Caches/ and Originals/
(Select all the folders in them, right-click, delete)
Voila. I got my 2.75 GB back. And when I re-loaded the Music app it a) worked correctly, and b) did not use of 2.75 GB of artwork cache. I did this with the free version of iMazing, which is pretty...amazing. Kudos to the developers.
I reported this bug to Apple through official channels in fall 2021 and never heard of a fix or resolution. This is my fix. YMMV of course.