Would the best solution to this be rather than to add additional fields (and in the future features may be added/removed so would require changing tags each time) to indicate if a gifted sub was a mystery sub, to instead just add a reference fields that points to another id (in this case, the id of the mystery gift notification)?
This way with a reference you could easily know which subs are from which gift as they would all share the same reference, the message that they are referencing would have the details if it’s a mystery sub or not, and allow easier addition of new features in the future as none of us (or Twitch) would have to make any changes for the sub message code, we would just need to look at what’s being referenced.