Rob, it looks like that may just be culprit. In another drawing, I redefined an attributed block without the attributes and got the same result with a block that still has attributes. Attsync does nothing, doesn't even recognize any blocks with attributes. I can still edit the attributes, though.
Weird... I've never seen this before. I guess it's just something I've never done before (the drawing with the issue was not created by me).
Thanks for looking.