My spine file contains a duplicate bone entry in the dopesheet at the very bottom after all other bones. The runtime appears to be seeing and using both of them as one of them contains a bad keyframe at the end, and that keyframe was causing a pop in the runtime until it was removed.
I'm not sure how my .spine file got into this state, but it's the second time this has happened. Typically the .spine file is edited my myself and my animator going back and forth through Perforce.
From what I can tell in the file history this time it first appeared after duplicating the animations and hierarchy for the male character and then modifying everything to become the female character. File history also shows that the extra duplicate bone changes over time.
I'm happy to provide any number of states of the .spine file for you guys at Esoteric software and will send to your email.
Screenshot attached.
