I haven't noticed any broken Album Art. The files I'm working on are newly ripped.

I will try that to see if it fixed it for me, though I will add that it was the conversion from MM3 to MM4 that broke the album art for alot of my albums. I know this is a fact because I made sure my library was 100% clean before converting over to MM4. All my art is embedded also.

Enabling/Disabling the option fixed it for me, although I haven't tested on any further albums.

I can confirm this behavior on my Win7 machine, this has been on my todo list to investigate but haven't gotten to it yet.

I have an Album where some tracks have an embedded Art and some have no Art associated. I select one file open Artwork tab, make sure Apply to all files in the Album/Series is enabled and click OK. The Art isn't added to the files without Art.