I would expect that the OP1 does not send midi clock when in sync mode. And I have not experienced that behavior. The rk005 works fine for me here. With Octatrack as a master and OP1 as slave. The rk005 also can filter some events. If i remember correctly.
So to confirm, turning off sync on OP-1 did the trick for me!! Bloody hell. What a pain. That was the culprit. Actually I confirmed that when sync was off, it did route midi clock via my box to the connected device. So all is well now. The $60 MIDIPLUS box works great now