Yes - your are right - it's redundancy. Probably some change to the code to evaluate it was made is why it appears to work differently.
The sketch starts off unconstrained due to the loose end of the line, but it already has an overconstraint that the front end can't detect or won't report because of the underconstrained line. Once each DOF is constrained per element, then it can check the overall constraints, which is when it finds a closed loop of mutual constraints, which it lists.
The horizontal lines remain horizontal because they are each tangent to a 180 arc segment, which is held that way because of the forced symmetry by the horizontal constraint on the partial arc on the other end, and so forth through the sketch. If that shared horizontal alignment constraint is removed (vertical dash marks towards the right) then Sketcher Intent Manager will either float some other constraint in place or add a dimension.