That happened to me with about 8 juices back in October.
Replace that o-ring and stop using that juice in it or it'll wreck the replacement.
EDIT - I just saw that you already used the replacement. It's the juice.
Uwell said they were setting up a website to sell replacement parts.
They sent me a fistful of replacement o-ring paks so I don't know if they followed through with it.
I quit vaping tank crackers because if the rubber's breaking down then ewwww vapin' it.
I use mine at 75-80W and it hasn't been a problem since I quit using those specific reactive juices
It happened to the top one only.
I think it's because I top fill so it gets compressed, then released, etc
That must help it absorb the citric acid that some rubbers (and plastics) react to.
I checked with dial calipers and the rings grew .015-.020" ID & OD IIRC