I’ve checked in a fix to Dev-Framework for this issue which will make it’s way to main shortly and be a part of 4.13 (CL# 3045344).
The issue is that the body setup was not being updated when editing the tile map via code instead of via the editor, so rebuilding the collision does nothing. If you’re comfortable making edits to the engine you can make the same change (call TileMap->UpdateBodySetup in RebuildCollision if OwnsTilemap is true) but otherwise I don’t have a workaround for you until 4.13 comes out.
Cheers,
Noland