A new version of MAME is available that contains a bug fix related to the coco3 driver and the GIME. (You would see this as a blank green screen when launching a CoCo 3 with DECB). Big thank you to Tim Lindner resolving this issue!
Perform an “Update CoCo-Pi from git repo” on the Administration (Utilities) Menu and then install the latest MAME package (from the same menu). Select mame-0.236-prerelease-tlindner-1 as the default version and enjoy!