Try to click Apply Base in modifier's settings. See https://blender.stackexchange.com/questions/115661/2-issues-with-multires-modifier?rq=1
– Mr ZakJun 03 '20 at 23:07
Here's how it worked for me. I clicked on apply base while in subdiv3, then deleted the multiresolution modifier and then re-applied it. On subdiv3 it worked.
– IndividualGamesJun 04 '20 at 14:58