I updated the original comment to highlight what I’m trying to do.
I want to put a single trig on to the start of a sequence so that when OT playback begins, the arpeggiator sends MIDI out to external hardware (press AMP twice to launch MIDI ARP SETUP for reference), but that is not happening and I do not understand. I have checked the manual (chapter 15) but see no mention of this. I have also tried yours and others suggestions to have multiple notes to no avail. I now remember why I do not use the OTMKII.