Issue was caused by call ED_sculpt_force_update from WM_file_write which frees
derived mesh and pBVH and forces them to be re-created. After this sculpt session
wasn't repairing properly.
Freeing data from WM_file_write is not nice by itself, but it exposed possible
real issue when sculpting on modifiers. Made it so BLI_pbvh_grids_update will
update pointers to material flags and hidden grids as well.
This isn't idea solution for particular circumstances from the report, since
pBVH would be rebuilt after save which ends up with small interface lags for
while pBVH is rebuilding.
Would be nice to have multires displacement propagation to higher levels
without freeing data.
Sergey Sharybin
eacc74ef1b