Unfortunately there is an issue with Reapers' and Bitwigs' Mackie MCU Protocol. 
It seems to be a common problem for Mackie users with Reaper...

Yet, We have found an alternative!

Thanks to; https://www.mossgrabers.de/

This is a standalone program that will bypass Reapers Midi settings, and instead runs through its own standalone app, 'moss grabber'. (so midi in and out are disabled in Reaper settings).

Follow the Installation instructions.
Installation Video: DrivenByMoss - Installation for Reaper, Documentation, etc.


ONE Control may not show a connection but once you move a fader everything will sync in.