The issue isn’t that the both compile, it is that lightmass gets the wrong value. I made a simplified reproduction with purely diffuse. The material instance ends up reflecting red light instead of green light in lightmass (ground is white in image), even though it sets the switch boolean to be green and in the scene it renders as green:
](filedata/fetch?id=1771454&d=1591326436)
(edit: fixed here: https://github.com/EpicGames/UnrealEngine/pull/7025 )