it does work, but are you assuming the plocks behaviour is the same as on an internal track, it is not !
if you plock internally you will get this (one p lock on 5 of 8)

_-

externally you get this

____---- (that’s just the first pass)

to get the same you’d need to plock on 6 to bring it back down

may not be your issue of course, just a thought

i guess you have the midi cc stuff sorted and working before locking as you say