Vanilla fish, tadpoles and dolphins "collide" with any non-water block 1 block above their position

Affects: cod, salmon, tropical fish, pufferfish, tadpoles, dolphins
Does not affect: guardians, turtles, frogs, drowned

The affected mobs react to any non-water block 1 block above their position as if they are colliding with it. This causes several pathfinding problems.

Steps to reproduce

1. Load the attached test world. [mediaInline] It contains a test chamber with a small water gap between two sides with deep water. Buttons spawn vanilla and custom fish and kill them, and levers toggle the blocks below the gap between stone and water.

2. Press the button to spawn a cod or tropical fish on the left side of the tank.

3. Watch the fish until it moves through the narrow gap to the right side.

4. Continue watching the fish until it tries to move from the lower half of the right side through the narrow gap to the left side.

5. Repeat steps (2) - (4) with the “random fish”, which is a custom mob with 0.5x0.5 collision that has the same navigation and random_swim components as the vanilla cod. It appears as a green cube.

Expected results

All fish move smoothly through the gap in both directions.

Observed results

When the vanilla fish move from the left side to the right they jolt downward after going into the gap under the center wall. When the vanilla fish attempt to move from the lower right side to the left side, they get stuck at the gap trying to move into the edge of the stone. The custom “random fish” does not have these problems.