This is not the right fix, but it is probably the safest one for 2.91. In order to fix this properly the steps would be:

• Commit D9079, which includes a function to check if the PBVH is rendering in flat or smooth shading
• Tag all tools and operator depending if they need normal updates, normal updates per redraw or normal updates on finish
• Add constant normal updates to the main brush loops for tools that need high precession normals
• Make only smooth PBVH shading update the normals on redraw.

The goal was to have a single call to BKE_pbvh_update_normals. This change adds one to a seemingly-generic place, but there is still BKE_pbvh_update_normals in drw_sculpt_generate_calls.

For reference, the issue is that in the main case (pbvh drawing) we have access to the object just before issuing the drawcalls (and after culling and / or visibility testing). But for regular drawing we don't. So we have to delay this to after all the drawcalls have been registered and the batches queried. And in this instance the batch can be lazily generated but not for pbvh drawing. So we need to have BKE_pbvh_update_normals in both places.

Discussed with Clement why the seemingly-easy-and-proper-fastest approach can't work sop simply.

To summarize:

• We only want the BKE_pbvh_update_normals to happen when object changes (so that camera navigation does not trigger the update)
• Multiple engines can require PBVH normals to be up-to-date
• There is no parent call in the draw manager which has all access to needed data AND will know that PBVH normals WILL be needed.

I can't unraise the concern, and accepting it at the current state is also not something what should be done. So resigning and leaving the final review/greenlight to Clement.

Note that I've modified the comment to reflect my changes. The update does not happen on view update now.