The player and other entities automatically jump when falling into water of any depth from certain heights. This was first introduced in 1.13-pre5 (it does not happen in 1.13-pre4), possibly from the fix to MC-132064; however, the similar MC-135211 also happens in earlier versions.

This differs from MC-135211 in that it can be reproduced with different heights, and can happen with water of any depth. Notably, it happens when walking from blocks level with the water into the water (one jump), and blocks 1 above the water (the player continuously jumps in this case, which is very strange). It does not happen for the 6-block drop, nor for the 1-block drop into flowing water (level 7).

Armor stands are affected if given force (/execute as @e[type=armor_stand] store success entity @s Motion[2] double -.1 if entity @s). This does not seem to happen with lava.

To reproduce

1. Download water_fall_auto_jump.nbt
2. Load it into the world
3. Go to the orange layer in the center
4. While looking very slightly to the left, hold forward so that you slowly move towards the edge
5. While falling, still hold forward
6. When you enter the water, you will automatically jump, and continue jumping until you stop holding forward or reach land.

Additionally:

1. Go to the white layer on the left.
2. While looking very slightly to the right, hold forward so that you slowly move towards the edge
3. When you enter the water, you will automatically jump once