It's actually kind of hard to know for sure what went wrong without being able to examine the original STEP file and seeing how other programs also behave with it as well.

That one particular area has a trimming boundary that looks like this (note the pretty big gap in one area):



That doesn't quite look like a seam splitting problem, it looks more like bad/inaccurate trimming geometry in the original model, but it's hard to know for sure without going into that area in NX in the original model and examining it though.

Is it possible for you to send the original STEP file before import so I can test it with a couple of other programs to see what they do there?

- Michael