Well I found the easiest way to reproduce the bug for you, have your terrain use 2x2 sections paint on it for a bit sculpt it a bit, then change it to 1x1 with resample on, this terrain was 256x256 with max res.
This image shows that the 3 layer terrain surface is corrupted by blending all 3 of the materials together at 100%.
You cant visually see that I cant paint but I could not paint.
Meaning if i was painting it would not update live if I saved and closed the editor then I would see what I had painted during this broken state.
I could stream this for you on Twitch if you want took me less then a minute to break it. Let me know.
