That’s most likely not it, or even related to it. Asset placement editor mode seems like beta version of the mesh paint mode. It’s literally just a subset of Mesh Paint mode, or in other words, worse version of it 
As the feature name suggest “Rule based placement” means the assets are placed based on a set of rules, not painted manually (at least not primarily. So it will be something else. I remember reading that it was Houdini related though.