I posted this on the Corona Forum as well, because I'm not sure if it's a Corona or Forest issue (or both): https://corona-renderer.com/forum/index.php/topic,15379.0.html
We have a scene with a forest pack pro scattered over a plane, with Limit to Visibility enabled in the Camera tab of the Forest object. The material of the geometry loaded into the forest is a Forest Color. The Tint Override tab of this Forest Color is Enabled, and we have it set to Get Color from Map. In here we load a diffuse map (a simple PNG). It is set to "As texture on Surface".
When we render, if we do not move the cursor AND we keep the cursor on the VFB, everything renders fine, for as many passes as we want. However, as soon as we move the cursor off the VFB and over ANY viewport (NOT any other part of the max UI, JUST viewports), what happens next is strange...
The forest geometry appears to render correctly, as in it doesn't change, however the map that we have in the Override "As Texture on Surface" slot is somehow... wrong. Like the UVW map is wrong/changed and the colours on the forest are completely incorrect. This starts to creep through into the render as the passes resolve.
The issue is solvable by DISABLING "Limit to Visibility" on the Forest object in the Camera tab. However this is going to cause major headache with ram and render speed of course...
So just to summarise, the bug appears to be with Limit to Visibility and Forest Color Tint Override. When LtV is enabled it looks like the UVW (or something else..) for the tint override map is somehow ignored/wrong, and this is all driven when the user moves their cursor off the VFB and onto a viewport.
Latest corona 1.5.2 and latest FPP, max 2016.