I've followed the instructions on adding labels to the rich text editor via the JSON file which worked fine, but now all the slices I added to that custom type have disappeared and can no longer be added to the custom type from my localhost slice machine. Adding the slicse back in via the JSON file also does not work, as in order to push the custom type back to Prismic I need to save to the filesystem which then removes the slices as they cannot be added form the slice machine UI. I'm kind of stuck here as I wanted to update my live site, but now any updates will take content out of the site.
EDIT: Just to add to this - can't even add any custom fields to this particular custom type. The UI won't allow me to type into fields at all and feels really buggy.