[DaveBlake]

I can confirm that. The metadata is read even without the call to FFmpeg, surely it is read from the DB. But since replaygain is not stored in the DB, without the call to ffmpeg it is never picked up.

Yeap.
If the item is not in the db then the backgrond loader does fetch the tag data including replay gain, but when it is in the library if just populates the item CMusicUnfoTag from the db . This does not hold replay gain.

what confuses me here is the need to read out rg in the codec. it should not be necessary and smells like an old workaround for a bug. it originated in 43e1077dadd0da0d5954768f9ece926c559d8371
as in the data should be attached to the passed item already.

Not if it is a library item. Not my design, but that is the reason for what it does as far as I can tell.