Thanks - I can reproduce this too (only occasionally) and for me the 8 mm Horizontal Distance Constraint doesn't only disappear - it gets deleted.

After dragging the vertical line segments and then applying the 8 mm Horizontal Distance Constraint the Solver correctly reports that I have 1 DOF remaining. If drag one of the vertical line segments (I used the right one), the Solver incorrectly Fully Constrains the Sketch. Now, while sometimes after dragging a bit more, the Solver correctly reverted to Under-constrained with 1 DOF.

It sure is a weird bug...