Ok... Went back to basics this morning
My starting point is I have two PRVs open, same track and settings assigned at this stage, one is displaying controller handles correctly the other isn't.
I inserted a completely new MIDI track assigned it to a separate softsynth and then opened it in the 'non-handles' PRV. Drew some CC-1 data, and...dum dum dum... No handles show.
So I closed both instances of the PRV and then opened the track created above in a fresh PRV and...ta da... Success controller handles are showing.
Lock that PRV and open a different track/synth combo in another PRV, handles show again, which has just completely blown my "2 PRVs open at the same time is the bug key" theory. I thought I had the recipe for a minute but no, I'm more confused than ever now.
I've unchecked the "show controller handles" option and now I can't get it to NOT show the handles. See below..................
Note velocity data appears in cc64 data lane. At least that bug is consistent.
I'm going to scream........ close the PRV and record all my controller data in future.........