The root of the issue goes to the discontinuity between the way how
mesh_calc_modifiers() and BKE_sculpt_multires_active() works.
At some point detection of original data usage by a modifier got
broken: the mesh_final based check is unreliable because deform-only
modifiers will create mesh_final for the connectivity information.
This made it so modifier stack evaluation would skip multires
evaluation, but the sculpt code will assume the multires is properly
applied.
This change makes it an explicit check about whether there are any
non-deform-only modifiers applied.
Pair programming and review together with Bastien, thanks!
Sergey Sharybin
ca1721270a