You could try generating a completely new AI grid, the AI grid and painted surfaces seem to be saved as part of the same file. Unfortunately this will probably remove all the surfaces in the level, meaning you'll have to repaint everything including your new surfaces.
I'm not 100% sure this will solve your problem, but I had to full generate the AI grid for For Joy as part of a mod I'm working on and after doing so all the surfaces I paint in the editor do show up in client.
I'm having a different, yet similar issue. I modified the terrain (both topography and material) and instances in the editor, but for me those are the modifications that revert in client. I'm trying to figure out why they aren't working but its like beating my head against a wall at this point.
Anyways, hope this helps.