Changing closed curve starting point and getting a different curve shape

(posted originally in the wrong category, reposted here…)

Struggling with this. Why???

RelocateCurveEnds_funnybehavior.gh (38.1 KB)

I can understand that the RelocateCurveEnds put just the starting point in the instructed position, and all the following points remained in the position they were before the RelCrvEnds comp (I 'd expected that they would shift accordingly, following the “movement” of start point); in the process of connecting the starting point with point #2, two other points were created in the interpolation.
If group 3 (containing the RelCrvEnds comp.) is connected to the orginal curve all seems to proceed well, except that when it reaches the position where the previous end point was, the sequence ends and short-cut abruptly to the new starting pooint (and the remainders of the curve are thus discarded).
Almost the same if group 3 is connected to group 2: mission aborted at the old ending point.

But - again - why this?

Is this related to some nasty point order in the original curve? To some misinterpretation of a single component? To my processor?
I’m specially able to create strange things lately…

BTW: I’m interested in understanding the logic of this (badly) returned curve, not just to have this particular curve set & combed (I have other curves to set like this, and then tween them…)

Just to complete the picture here:
I was trying to set a closed curve in order to have the starting and ending points perfectly set in the Y axis. I choose RelocateCurveEnds for this 'cause I’m not confident using Seam.
But didn’t exepcted it would be such a pain…
I’m stuck and run out of ideas.
Any help?