Replay Gain is not...

[pl_svn]

... saved to AAC tracks :-/

(calculate RG, save, quit Tag, relaunch and... no RG)

btw: Cover Art's "Type" too is not retained and always reverts to "other"

[sbooth]

I assume these are AAC files embedded in MP4 (.m4a) files?

[pl_svn]

yes: Apple .m4a AAC

[sbooth]

I've fixed this problem- thank you for reporting it.