I’m not sure about the exact back-end mechanics, but characters are inputted by combining a character with a modifier, for example to produce the symbol ɛ you would type e followed by <. It seems that Keyman inserts a backspace to delete the original character and then replaces it with the modified one.
However, in Dorico, doing this results in eɛ. Keyman works flawlessly everywhere else in MacOS. Every dialog including popover dialogs are affected. I’d be curious if this is the same in Windows.