@udo-audio Appreciate that you are looking at it!
I also filed this related request which I think should be considered a bug in how the external midi control works.
Generally the sequencer has 2 modes, the transpose mode and the mode where you can play additional notes over the running sequence. I find the second one to be much more useful.
But when a sequence has been started by external midi message, there is no way to have it running in the mode where you can play additional notes on top of the sequence. It would be great if this could be enabled.