Hello gunspike, What you posted is the normal behavior of paths in Spine, not a bug.
The two straight lines are a visual indication of where your character will end up to if you go over 100 with the path position.
They were introduced in 3.6.49 so that "Paths that are not closed show straight lines for before and after the path when selected."
It's a relatively new feature so I understand why it might be a surprise for you (:
