I think I understand what you are saying, but I'm not sure. I think you're saying that if you make the bottom end of the spline perpendicular to the vertical construction line on the right, you get an error.
Yes. This is true. Splines have a different way of working that doesn't seem to follow the same rules as the other sketch elements. If you select the spline and the vertical construction line on the right, and assign a perpendicular relation, you get an error. But if you select the spline handle connected to the bottom end of the spline, and the vertical construction line on the right, you can make the perpendicular relation correctly.
Notice below that the diamond on the lower spline handle is selected, as well as the vertical construction line. This will work, as selected. Don't pick the spline directly.