Can anyone explain to me what information I am leaving off this sketch?
The spline sketched below should be symmetrical, or more accurately, the ends should be as though they are rotated 180 degrees around the center, but they’re not. The little inset in the bottom right shows what it looks like when I mirror the sketch left/right then top/bottom. It should lie on top of the original but it doesn’t. The dimensions should make the thing symmetrical but don't.
For splines, there is position and angle but also the influence drag handle. You can mirror the position and make the angles the same, but making the influence drag handle symmetric is more difficult.
One way is to make half of your spline and then use copy > paste special on the sketch entity to create a dependant copy at 180 degrees. This has the advantage of leaving the spline drag handles "dragable" in your sketch.
Another way is to select your spline in the sketch, RMB and pick "modify" and on the spline dashboard pick the icon that shows the drag handle dimensioned. This requires you to fully dimension the spline, including the drag handles, however you can then make the drag handles equal and apply constraints like symmetric to the drag handle intersections.
I rarely use splines, so sorry I don't have an answer, but I was trying to learn more about them. I drew this using the polygon control, but it still doesn't look 100% correct as there is a drag handle slightly off center and I'm not sure why.
Looks like you're missing an on entity constraint in the upper right to the line indicated.
It's not immediately obvious to me where the problem is. It looks OK to me, have you tried rotating a copy to check symmetry?
I'm stumped, I assumed that locking down the control polygon would define the spline explicitly.
No time to play with it right now, I may be able later.