Cesium for Unreal filters out small triangles when constructing the physics mesh. We do this because we’ve otherwise seen Chaos complain about these small triangles and then collision doesn’t work on the entire tile. The code that does the filtering is here:
It could be worth commenting this out and see if it helps in your case. The plugin will need to be located in your project (rather than in the Engine/Plugins/Marketplace directory) in order for code changes to be picked up.