Congrats! So we now know for a fact that it was MIDI clock causing the problem.
I was pretty sure of that after reading in the Keystep manual how the damn thing would echo any clock-type signal at the MIDI in to the MIDI out.
All of that control in the software editor and they left out the ability to prevent that… You should take a minute to drop them a line!
So, you found a way to make the Studio 4 do exactly what I was suggesting you do with an external filter.
What's remarkable to me about that is having used Opcode Vision, StudioVision and a Studio 4 for years and years, I have never once seen those little extra pop-up windows in the OMS Routing panel. Apparently, I never needed them!