Thanks to all for their replies. I've done a bit more testing. Results as follows:

1) Changing styles seems to make no difference at all (using all RTs/RDs)
2) The holds problem is with piano RTs. Different jazz bass RTs play nice holds on the root note.
3) I'm not certain the piano holds are midi - but they are louder, aggressive and don't 'sound the same' as non-hold RT chords. For example
- Jazz piano RTs for Cmaj7, C6, C69 - all with holds - play the same 'insistent and loud' C triad
- And holds with Cm7, Cm9, Cm11 all play the same 'insistent and loud' Cm triad.
4) Trying different piano RTs (bossa and swing at different tempos) I get the same insistent 'block triad' sound with any held-chord - which is what makes me think it's midi, but I may be wrong.

I wonder if BIAB staff could comment on how jazz piano RT holds are supposed to work - or if there's something wrong with my set-up.... ? (As I said in the first post, I think I have everything up to date.)

Thanks again to all. Best - Andy