Drowned and players can attack, and players can break blocks, through waterlogged upper slab

Possibly caused by MCPE-31896, based on the screenshot showing the trident hit the chest on top of the slab.

I don't think so. My character was not fly or swimming. The trident you see was thrown by a drowned while standing on that chest. It would have caused damage my character if it was standing on the slab floor. The drowned can see, follow, and damage my character standing on this slab floor.

Yes, I meant that the drowned was swimming directly under the slabs and that made the launch point for the trident be above the slabs. In other words, I think MCPE-31896 applies to drowned just like it does to players.

I was not able to reproduce this issue, tried on flat and standard world, standing on chest and without it, with different slabs. Best results i got was Drowned following me with their eyes. Could you please provide a wider angle view of the structure that the issue occured on or a video?

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Here is a video. It shows my character being struck twice by tridents from drowned.

https://youtu.be/P5OmXMuvN0M

I was mistaken, this is not related to MCPE-31896. Indeed drowned do not even shoot tridents when within 3 blocks of their target.

The bug instead seems to be that the slab collision is not calculated correctly from certain angles. Both drowned and players can shoot tridents through upper slabs from underwater, and players can also break blocks behind upper slabs. Here are demonstrations:

To interpret these videos it is important to note that the tridents that hit the mobs appear to hit well below the slabs and fall down below the slabs only because of MCPE-109423. That bug apparently extends their projectile hitbox in the direction of movement such that the hit registers while the center of the trident is still below the slab. However, that is not the cause of this bug. This bug is that the game allows the trajectory, not just the hitbox, of the projectile (or the player's block-breaking reach) to pass through the slab. The fact that tridents can pass through the slabs and lodge in the chest above the slab proves this. (If there are no blocks on the other side the trident flies through on a normal trajectory; while testing I launched one dozens of blocks past the chamber.)

EDIT: the crossed-out text above was based on an incorrect explanation of MCPE-109423. That bug is caused by the fact that when the game calculates a projectile hitting a mob, it keeps the projectile's position as its last ticked position instead of updating its position to the edge of the hit entity's collision box. Tridents move 4 blocks/tick, so they can "hit" and fall up to 4 blocks away from a mob.

Breaking blocks behind slabs is interesting because if there are no blocks within reach behind the slab then the slab can be broken as expected. This suggests that the game is offsetting the position of the slab's collision box.

The bug may only occur while moving upward or downward in water. I only had problems breaking slabs while allow myself to float instead of using flying to stand still.

I am not sure if this bug can be reproduced without waterlogging the slabs, or without the shooter being in water, as I only tested the reported scenario. However, I think this bug may have the same underlying cause as MCPE-106860, where ghast projectiles can pass through bottom slabs when shot from above. And snowballs fired by snow golems have reportedly exhibited some form of this behavior: MCPE-125598.

Still occurring on 1.19.63