I’ve dug so deep into this issue before and I’m still yet to find a solution, I will however elaborate on what I’ve found. And another solution if needed.
It’s basically a gimble locking issue and it occurs when the spline forward axis at a certain point is equal the the direction the spline is going. What I normally do to tack this problem, is to always build the spline on the X axis, then just rotate the actor instead (or whatever you parent you spline to) to get the desired orientation.
Obviously that doesn’t fit the bill for all cases, but if what you are making doesn’t need to move it’s much easier to build complex shapes that way.