I looked at the Sorting options and none were ticked but all the albums had iTunes compilation flag set to 1. Removing this restored everything to normal. As I have never used iTunes I wouldn't have spotted this in a million years!
Glad it worked!
If the "treat iTunes compilations as" option wasn't ticked, that shouldn't have been happening, though. If you can document that for sure, it's a bug.
The flag gets set by many taggers, including Picard, partly for compatibility.