Maxwell Render

Hi all,
I had a problem and found a way to get around the problem but I can't really solve it. Can someone help me understand why merging causes this change in tangency?

I imported a cylinder, detached at the center lines (isoparms), into Maxwell and when I merge the data the cylinder was no longer tangent at the connecting edges (isoparms). When I opt group instead of merge, the cylinder is tangent.

I am importing from Alias Studio into Maxwell.

Could anybody explain why this is happening?

Thanks in advance for your time.
Giuseppe