Unsure if this is intentional design or simply an oversight, so I think it's worth reporting.
Before 22w12a, Wardens were considered undead and thus didn't drown. This could be considered an anticheese measure. 

When added in 22w12a, they stopped being undead and thus were able to down again. 
I think perhaps it would be safer to see if this is WAI, as it was never specifically stated whether or not Wardens were meant to be able to drown.