Bit of a strange one - I can only get my external keyboard to trigger a sound on Halion 6 standalone if it is set to receive on midi channel 16. It appears to receive midi signals on other channels as the monitoring light flashes when the keys are played - but no notes triggered.
I had the same problem with Halion 5 but left it as I hoped it would be sorted in Halion 6.
I don’t have this problem with Groove agent 4 stand alone, Kontakt 5 or any of my DAWs (Ableton, Cubase) Dorico etc