@ian-o : I had this same issue with the UNO. It would sometimes work with midi out/in and never worked for clock. I got a Roland UM-1 and all the issues went away. I chalked it up to the UNO just not having full compatibility with the original organelle. I have no tech sheets or specs to back this statement up; only personal experience.
My advice? ditch the UNO and get the UM1 if on a budget or RK005/006 if you want to supercharge the setup.
(i have not tested the MK2 but mk1 works flawlessly)
You can find this used for around $30 and new for around $45… Also there are two switches on the UM-1 mk1 and full functionality works with the “advanced driver” switch in the ON position. The other switch is midi thru… Seems the mk2 doesn’t have switches though…
Both Retrokits devices have advanced filters and routing options that will make it a much more powerful and flexible midi tool. but can also just be a simple 1x1 midi interface over usb. These both cost alot more thuough so it might not fit into your “cost effective solution”… for that i’d go for a UM1.
Hope that helps a bit.