By default, beets won’t touch the artwork in your files.
The only real interaction beets has with album artwork from my understanding is via the fetchart and embedart plugins, which are used to download artwork and embed into files it respectively.
If your box set is recognised as a single album in beets (which it most likely is), beets will only be able to manage one cover image for the entire box set. This doesn’t prevent you from putting your own cover files in the disc folders, it just means beets won’t manage them for you.
This sounds like a feature request then - it’s probably worth opening a GitHub issue for this. #231 seems somewhat related, but it doesn’t specificially ask for per-disc artwork, just that we handle album artwork better when there are disc directories.
Do you have an example of an album on MusicBrainz which has different artwork per disc?
This is unusual, most of the boxes where individual volumes have a unique cover, Original Jacket style, there are no individual covers uploaded.
Even if the were, there is no relation ship I know of where you can link an individual Volume to single art, so how would beets know which uploaded art is a cover for a which volume ?
There might be a work around. One of the plug-ins mentione above, allows to be configured to pick up covert art from album directory and then even delete it if required. Thing I don’t know is, if this works on individual folders or its done on the album level.
I think @nadl40 is right here regarding the relationship - I can’t see any clear correlation between the data returned by the Cover Art Archive and the disc that the artwork belongs to, so I don’t think we’ll be able to match them up (unless the ID in the Cover Art Archive data can be used somehow?).
This would mean we can’t support this configuration for fetchart, but I think beets could still support per-disc artwork otherwise?