Pasting SVG seems a bit broken. I have 1778 curves that I've copied from Rhino 3D WIP (it copies as SVG). I paste it to Illustrator, and Most of the curves automatically change to straight. Not only that, but it's automatically merging some of the points that are close together.

This leads me to believe that Illustrator is rounding the points lower than it should.

Enter Inkscape. I pasted there, and it's fine. I wanted to check to see if it was a failure of Rhino. So copied from Inkscape and paste to Illustrator....same issue.

Copy only a Little of the same file from Rhino. Oh, all the curves are fine. Copy more, Some lines are curves, and some lines are straight.

One more test, and this one might reveal the issue: I copied just a few lines from the outer areas of the entire object I'm trying to copy. The same issue happens, where most of the lines are now straight. My guess is that Illustrator is automatically changing the number of decimal places stored, based on the bounding box of what is copied. Well, that's wrong. Please don't do that.

A reminder to Adobe folks. SVG is more than just for icons.