Okay, after a lot of looking around in the many different states (effects). I have one last different idea on what it could possibly be.
I think it is drowning. The reason is because you claimed Ace and Luca fell. However, Ace was the only one affected (This makes me think it is drowning because Luca is immune to it unlike Ace). While it could be poison, you would see the icon for poison (looks like green gas). However, you said that purify did not work, I realized it could not be the poison effect like I suspected the most.
What I actually think it is after looking deeper into this, is that within RPG Maker, usually having the "resist (insert state)" tag makes it so it is impossible to obtain that state. I was under the impression that if I added that tag to the water breathing state, that it would simply auto-remove the state. However, it does not do that, it just makes it impossible to have it re-applied (so if you already were drowning, but have water breathing, you would still have it, but would not be able to get it again). The reason why this state wouldn't appear is because the drowning and water breathing share the same icon (I will change it in this next update to something else since I don't know why I thought that was a good idea when making this system). So what I am going to do is make it so the potion will also remove the drowning state since I am now aware that resist doesn't get rid of it (In the next update). In the next update, I would like it if you could test this again to see if I am correct. Thank you for your patience as I have now learned something new (Thankfully this was the only state I did this resist thing on so it is a very easy fix, shame finding it wasn't as easy)!