I have reported this to MKB dev but thought I would report it here in case there is something you can do on your end as the dev does not look very active. I will also be reporting to the mods that require it as a dependency.

Basically with both mods installed you cannot assign modifiers to keybinds. Instead the last key pressed in the set is set.

MC 1.20.1
Modern Keybinding Fabric-1.20.X-1.2.0
AMECS 1.3.10+mc.1.20.1
Fabric 0.15.11
Fabric API 0.92.2+1.20.1