I noticed this same thing but with water. Basically, if a fox jumps after another mob while both of them are in water, the fox will keep diving down to the bottom of the water, causing it to drown.
Confirmed in 1.20.1
Did you receive a normal amount of XP after the fight (usually enough to take a player to level 60)?
I noticed this same thing but with water. Basically, if a fox jumps after another mob while both of them are in water, the fox will keep diving down to the bottom of the water, causing it to drown.