I'm the first to ever mention this, could be added as optional if people desire >.<
"This is actually supposed to be a feature. When you reduce the radius to zero WorldPainter assumes you want to manually paint individual pixels and does not apply the brush (including the brush filter). Otherwise it might not do anything at all, or not paint solid pixels, if you have a non-solid brush selected, or do other presumably undesired things."
https://www.reddit.com/r/Worldpainter/comments/18pjsfn/can_anyone_reproduce_this_bug_brush_radius_0/
I'm the first to ever mention this, could be added as optional if people desire >.<
"This is actually supposed to be a feature. When you reduce the radius to zero WorldPainter assumes you want to manually paint individual pixels and does not apply the brush (including the brush filter). Otherwise it might not do anything at all, or not paint solid pixels, if you have a non-solid brush selected, or do other presumably undesired things."