While the input channel names on the SQ are recorded with the Scene data, it appears that the names of output busses (MIXes) are saved in the Show data. The FX Sends and Returns do seem to remember their names as the scenes change, which is handy if scene changes include different FX parameters.
Is there a logical reason for this that I’m missing?
Turned out to be Operator Error, not a feature of a bug.
I had some of the Mixes set to SAFE, which of course prevented them from changing. The whole recall filter/safe thing can be a bit of a minefield. Other than setting up some scenes to just change FX, I didn’t think I had engaged any, but I guess that wasn’t the case.