If I'm looking at it right, I'm thinking more like wear in part #'s 6, or 13. Possibly even the woodruff key #17 being work, or partially sheared. It may partially engage the gear, but depending on how much the external linkage is worn, and not push it into the full detent position. Or as mentioned wear in #'s 6 & 13 not fully engaging, even though it is in the detent notch.
Not sure how much that little finger on #6 would take, or how much wear has occured if it's been haunched on pretty good in the past.