Hello,
I’m not sure if it’s a bug or if it works “as designed”, so I’d rather report it. I’ve just noticed that AO doesn’t work correctly when used in CoronaLightMtl, as long as Emit Light or Occlude other Lights options are disabled … bug or feature ? 
3dsmax2018.4, Corona v1.7.4
Thank you.
Any one, please ?
We will look into this, just need a bit more time.
Can you provide more info? Screenshot? Scene? It always helps.
Hi Maru,
Sure, here is the scene and rendering.
On a side note, I noticed IPR is significantly longer to start when Light Material is set to emit light …
Thank you 
LightMtl with “emit light” off is invisible to the GI (does not cast shadows, generate light, etc), and that’s why AO is not working with it.
If you need such exotic setup (for any reason), use RaySwitch material and plug the LightMtl with emit light off into the direct visibility, and plug any shadow-casting material (such as standard Corona Mtl) into the GI slot. This way it will magically start casting shadows, will not generate light, and the AO will become visible.
Oh good idea, Maru, I’ll try your workaround, thank you !
With your explanations, it’s clear now … not a bug then 
Cheers.
