In our project we have an issue where several of our older animations are missing a series of slots entries. When transitioning between another animation to one of these there are several pieces that don't properly update and end up appearing very morphed. Going into the json and manually adding what should be the missing slot entries does resolve the issue. In spine itself it is very clearly setting the appropriate slot & attachment. Freshly made animations do have the expected slot timeline, however attempting to copy-paste the keyframe data from those onto the older ones did not resolve any issues.
I'm not sure if there's some default settings at play here or what, I've only been able to find a tangentially related bug from 5.5 years ago: http://en.esotericsoftware.com/forum/Attachments-missing-from-exported-json-3979
