Hi Jacob,
In my admittedly limited experience, the preamp gain control will not display for purely digital signals. Were you using a digital or an analog source on the affected channel strip?
I’ve also experienced issues with ME-1s that were on an older version of firmware, especially in terms of inheriting stereo linking, which doesn’t seem to be your issue. Are your ME-1s and ME-U running the most recent firmware? Or perhaps is that ME-1 on a different version of firmware than the rest of your units?
Also note that ME-1 units can be daisy chained, but each unit after the first one would require its own power supply, as only the first unit can receive power from the ethernet cable.