I looked at your project and see that your collision meshes use the “Use complex collision as simple” setting. This cannot be used since the generator uses these models to detect overlapping tiles. To do that, the collision meshes need to consist of convex primitives which means you need to use the “Default” setting and add collision primitives.
There are multiple ways to add collisions: https://docs.unrealengine.com/en-US/…ttingCollision
The best way to add collisions are custom collisions: https://docs.unrealengine.com/en-US/…shes#collision
Make sure the collisions do not reach to the edge of the tile otherwise a tiny bit might overlap with the neighbor tile and will be rejected.