I thought I'd jump in here - the folks at VSL had a look at my template and impressively also figured exactly what Clintone has found, that having anything external coming in to Logic that is Input Monitor enabled, then your MIDI playthru monitoring is not working on any external MIDI, be it VEP or external MIDI tracks. Take a shot at this with my template - connect any of my VEP instances (all the orchestral stacks) to whatever instrument using VEP and you'll find there is no MIDI monitoring going on. However, delete my stack called 'LIVE TRAX' which is the external inputs from Ableton's Live (using Rewire) and you'll see it suddenly starts working correctly. That's because those are external tracks that are Input Monitored enabled. You'll see in my 'DIALOG,' 'TEMP' and 'PRINT BUS' tracks on top as well as the 'STEMS' stack those are all Input Monitored too but this doesn't affect the problem as those are all internal tracks, not external as is the case with the 'LIVE TRAX' stack.
STUDIO TEMP JAN 2023 v41.zip