I can't really reproduce it - but it sometimes occoures when rendering something.
Corons 1.5 hotfix1 and 3dsmax2017/2016.
When playing with Tone Mapping within the Frame Buffer sometimes when Rendering is finished the Output/Frame Buffer picture changes brightness dramatically....
In some cases i tried to alter White Balance and got a complete darkened result - sometimes i tried only playing with highlight compression or other values within Tone-Mapping and the FrameBuffer becomes dark - like some gamma values were dramatically altered.
It is not really reproducible - sometimes this happens, sometimes not - maybe someone else has figured similar behavior.
I attached 2 images showing the difference - the bright one was the original rendered image - and during rendering you could alter the tone mapping values normally - but sometimes (mostly after rendering) changing either value will result in darkened output.
But i can't be sure if its really just while rendering was done, or also happening during rendering in those cases, where this appeared. Usually i recognized when rendering was finished.