@orbitalzones is right, the issue is that unreal relies on a common spline up vector to determine how meshes should rotate about the spline tangent. While there are a few different workaround fixes, the only way to properly fix this issue is to replace the underlying deformation code in the spline mesh component itself and the shader with a new way of calculating up orientation without a spline up vector.
If anyone’s interested, I’ve released a plugin that does exactly that and working in UE 5.7