I'm not sure if this is consistently reproducible, but I definitely experienced it while Picard was getting a lot of 503s, and nikki mentioned it too.

Basically it looks like the code to workaround broken NAT releases is affecting regular releases too. When those 404, the files are moved and the broken release is removed. It's not checking the HTTP status code though, and also doesn't check that any NAT files were actually moved (those being files with "[non-album tracks]" as the album tag).