The bug

The drowned don't chase you even if you are right next to them in the water if the water is 1-block deep.

How to reproduce

1. Spawn a drowned into 1 block deep water (make sure the drowned is not a a baby)

2. Move around in the pool with the drowned (in survival mode)
   → ❌ The drowned will not chase the player

Code Analysis

@unknown - The following is based on 1.17 Release Candidate 1 yarn mappings.
The call to this.isTouchingWater() in net.minecraft.entity.mob.DrownedEntity as seen in the following code snippet makes the drowned attempt to use water pathfinding whenever touching water, which makes the drowned not able to pathfind as their water pathfinding is meant for swimming. Instead, this would be fixed by replacing that call with this.isSubmergedInWater(), as that checks whenever the entity is completely underwater which allows swimming and uses land pathfinding in one block deep water.

net.minecraft.entity.mob.DrownedEntity

public void travel(Vec3d movementInput) {
        if (this.canMoveVoluntarily() && this.isTouchingWater() && this.isTargetingUnderwater()) {
            this.updateVelocity(0.01F, movementInput);
            this.move(MovementType.SELF, this.getVelocity());
            this.setVelocity(this.getVelocity().multiply(0.9D));
        } else {
            super.travel(movementInput);
        }

    }