On an environment layer, the option's setting "Appears in Reflection" should toggle between Yes, No and Only.
That's how it is on PC.
On my Mac this setting propose only 2 values: Yes or No.
After loading a project done on PC onto my Mac, I see an environment layer with the setting "Appears in Reflections" set to "Only".
So it looks like AE does know this setting's value, but as soon as I edit the setting, Yes/No are the only values I can choose from.
A user on PC confirmed that this works just fine for him and he can toggle between Yes/No/Only at any time.
I'm running AE version 126.96.36.199
Retrieving data ...