The same thing was happening to me.
Try this:
Before launch session, select all grind rails, press (Ctrl+X) to cut, and then (Ctrl+V) to paste it again on the same place. That will be fixed, was the easiest way I figured.
My problem is another, maybe you can help me to.
First of all, I use UEFN on PC and creative on PS5, so you can understand.
I’ve created a gring rail (left) and I want to create the same grind rail on the opposite side (right), (rotating is not an option because will not keep the same purpose) so when I try to transform/mirror Y or X, it looks fine at the beginning but when I Launch session and connects to the console, always start a game first instead of creative mode. Everything is good here, but when I end game to start creating, my mirrored rail gets nots in almost every spline.
Only the originals stays normal.
Can someone help me with this?
Thanks