Ok, I have solved my problem and maybe that of some others. I have reported this as an issue on github as it means that either the manual or code needs to be changed.
Darktable uses channel 10 (on 0-15 numbering) as the channel to read from and channel 0 to write to (i.e as the controller’s global Rx channel). As received from factory the controller does indeed send on channel 10, but its global Rx channel is channel 11. Using the X-touch editor on a windows machine to change this to zero (in this editor this actually means channel 1 as it uses 1-16 numbering) all works as expected with the lights staying on when they should. What remains a mystery to me is why the lights around the controllers worked before when the wrong channel was being used. I did however notice when I was using the little command line program sendmidi to test, that occassionally sent messages on the wrong channel were responded to.
If the lights don’t work correctly for you I suggest that you try what I did. If you haven’t already got the editor it can be downloaded from here, but it does need a windows machine.