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
5e332fd700