Few questions regarding the poisoning thing.
1 - Does it only happen with the parasites?
2 - Does the purify spell dispel the damage over time thing? (Lotus, Ginseng, and White Tiger has it)
3 - Is there anything specific that you or the enemy does before the poison occurs?
4 - I checked Rei And Gillino, and nothing seems to be up with them. Who was the other person on your party? (If Otter or Carp (during quest) was also there, then let me know)
5 - Is this happening with Leon too?
6 - Would you happen to have the Shard of Melancholy equipped on someone? (Not sure if you defeated the new super boss yet) Since when equipped it slowly drains your HP and MP.
Sorry for all of the trouble I am putting you through with this damage over time thing, I am unsure of what it could possibly be. Since I double checked any new states or moves I used and they are all normal and I am unsure why it happens with the parasite. I hope at least one of these questions answers what is going on or at least helps me single other options out.