That’s a common issue. You can sometimes fix it by slightly changing UpDirection by using [Set Spline Up Dir] node in Construction, but with longer splines it usually just shifts the problem to some place else; maybe not with a 180 or 360 degrees twist, but it still distorts the mesh.
AFAIC, there’s no blueprint native way to completely fix this thing. There are some paid plugins that fix that, though.
I guess the issue is that blueprints don’t use quaternions for rotation, only Euler angles, so when your spline bend by 180+ degrees on one axis, this tends to happen.