I had heard a story about the Louvre, that when bombing became a possibility, the art was distributed to private citizens, and after the war, all of it was returned. While “all” seems unlikely, “most” seems possible. Perhaps France differed from Germany in this regard, since Germany was having essentially a civil war after their defeat in WWII.
Germany was different. Hundred of thousands works of art were stolen just from Jewish homes, 100,000 still missing today. I read a couple of books on the subject, the hunt to find these pieces of art is fascinating.