Mobs usually don't take damage if they fall into water "floating" 2 blocks above water, but in some condition, they do take damage.

In the attached movie, the ground is Y=56 and water is at Y=58 (58-59), and the player is falling into the water from Y=219, taking 1 damage. They don't take damage when falling from Y=218 or Y=220.

I tested different conditions. The player also took damage when falling from Y=235, 248, 258, 268, 278, 285, 309. 

When I changed the height of ground (and water), the problematic numbers changed relatively.  For example, when the ground is Y=66, the player will take damge if falling from Y=245, 258, 268, 278, 288, 295, 319. 

I think I have experienced this bug in some older version and so this bug is introduced not so recently.

Maybe MC-253212 is related.