This is a regression since PBVH was introduced for weight paint. The solution is: treat subsurf and multires modifiers as deforming ones for the weight painting. This is an easiest solution to make PBVH use subdivided location of original vertices. This change could simplify some of the weight paint by removing the grids check, since PBVH is not supposed to be built from grids in this case anymore. Differential Revision: https://developer.blender.org/D5751