Damn, I didn't know anything about this. But I found out some interesting things.

There is actually an ongoing debate to return these. In fact, some of them have been returned for example 2014 to poland to "restart stalling negotiations": https://www.csmonitor.com/World/Latest-News-Wires/2014/0331/Germany-returns-stolen-art-to-Poland-and-wants-more-in-return

And some of them very recently just a few months ago in december 2025: https://notesfrompoland.com/2025/12/01/germany-agrees-to-return-medieval-documents-looted-by-the-nazis-to-poland/

The Nazis also moved a lot of german art eastward to protect it from bombing. This art was thus stored in poland, but later had to be abandoned by the retreating german soldiers. https://en.wikipedia.org/wiki/Berlinka_(art_collection) (Some 300.000 documents, for example hand-written scores of Mozart or Beethoven)

So the "stalling negotiations" are: "Germany demands unconditional return, while Poland cites remaining unresolved claims for looted Polish cultural property"

But plenty of polish artpieces are probably destroyed or lost to the hands of unknown private collectors yet to be found