Why are 2 meshes lined up in modo and not in unreal?

Ok so the scene is set, I made a mountain and was told to split it up into smaller pieces. Why? I don’t know exactly, maybe so there are smaller drawcalls instead of one huge one. Anyways not important. So in one section I split them by using edge split and then exported them where they were. I didn’t recenter them so that when you place them in unreal you can grab them together and they will stay together. And then I noticed where the split was there was a gap in unreal but in modo I zoomed in as far as a I could and I didn’t see a gap.

Later down the road I was told to section off the mountain even more. So I did it differently. I split them up then centered them each then moved them so that there was a vert on the center so I could snap them together perfectly in unreal. So yes the pivots are correct. And in unreal they snap perfectly, except there is a small gap to where they are suppose to connect. Also this gap isn’t too noticeable, its a flickering seam like unreal knows its right but it says “no i am god and you don’t get to be seamless”.Anyways my fix was to crash them into each other and make it work for the camera since its a fixed camera.

This project isn’t the only time I have seen this happen and I was just wondering if anyone knew what it is?