
Thankfully, I have 3ds Max 2017 installed on my machine, and it still ships with mental ray, so I was able to test, reproduce, and work around the issue.

The remaining slider positions are also available as the Global Tuning Parameters rollout Soft Shadows Precision control. The leftmost slider position (“Soft Shadows off”) is equivalent to turning on Area Lights/Shadows as Points. Typically the light should be set to cast ray traced shadows, although in some cases shadow maps work too. This includes all photometric lights (Target Light, Free Light, mr Sky Portal), as well as mr Sun, mr Area Omni, and mr Area Spot.

A global multiplier for the Shadow Samples setting in all lights casting soft shadows.
