Hey,
i added a gif to this question where i tried to put a small plate on a bigger plate. As you can see the small plate goes through the big plate and the otherway around. I could not reproduce this behaviour in the engine so i would assume that the html5 version has a different handling of collision. Is there something i need to keep in mind when i want to place the small one on the big one. This bug only appears when i try to put the focus the center of the plate and try to put it there.
Your question mentions UE 4.16. Can you try with WebAssembly in UE 4.17? Collision handling should be the same, and there is a chance that WebAssembly improves the arithmetic related to that. (UE 4.16 was only able to do asm.js exports iirc)