[FP] Bobbing up and down at the surface of water sets you back

[snurre0]

Info

In Minecraft 1.21.11, if you use spacebar to stay afloat at the water surface, you will eventually get set back to wherever you came from before starting holding spacebar.
The easiest way I managed to reproduce this was by starting at the ocean floor, and then holding spacebar until I were at surface level, and then just keep holding spacebar until I eventually got set back to the ocean floor. If I kept holding spacebar I would get set back down again each time I got back up to the surface. The time it took until the setback happened varied a bit.

matrix log:

(21:01:14) SnuRRe0 failed Move check | unexpected vertical movement (motion: 0.142/0.131, hor: 0.0, buffer: 0.005/false, ground: -,-, setback: +, vel: , from: 62.606, past: -0.098, p_tags: liquid, tags: air_bst,liquid_vert_bad,liquid)() | vl:5(+5) ping:55 tps:20.0 component: move.vert ver: 1.21.X
(21:06:36) SnuRRe0 failed Move check | unexpected vertical movement (motion: 0.139/0.131, hor: 0.0, buffer: 0.004/false, ground: -,-, setback: +, vel: none, from: 62.468, past: 0.02, p_tags: liquid, tags: air_bst,liquid_vert_bad,liquid)() | vl:5(+5) ping:55 tps:20.0 component: move.vert ver: 1.21.X
(21:06:41) SnuRRe0 failed Move check | unexpected vertical movement (motion: 0.028/0.054, hor: 0.0, buffer: 0.013/false, ground: -,-, setback: +, vel: none, from: 63.029, past: 0.135, p_tags: liquid_up_x, tags: air_mdf)() | vl:10(+5) ping:55 tps:20.0 component: move.vert ver: 1.21.X
(21:06:49) SnuRRe0 failed Move check | unexpected vertical movement (motion: 0.142/0.131, hor: 0.0, buffer: 0.005/false, ground: -,-, setback: +, vel: none, from: 62.598, past: -0.098, p_tags: liquid, tags: air_bst,liquid_vert_bad,liquid)() | vl:5(+5) ping:55 tps:20.0 component: move.vert ver: 1.21.X
(21:06:55) SnuRRe0 failed Move check | unexpected vertical movement (motion: 0.145/0.131, hor: 0.0, buffer: 0.007/false, ground: -,-, setback: +, vel: none, from: 62.177, past: -0.097, p_tags: liquid, tags: air_bst,liquid_vert_bad,liquid)() | vl:5(+5) ping:56 tps:20.0 component: move.vert ver: 1.21.X
(21:07:02) SnuRRe0 failed Move check | unexpected vertical movement (motion: 0.145/0.131, hor: 0.0, buffer: 0.007/false, ground: -,-, setback: +, vel: none, from: 62.177, past: -0.097, p_tags: liquid, tags: air_bst,liquid_vert_bad,liquid)() | vl:10(+5) ping:55 tps:20.0 component: move.vert ver: 1.21.X

Video

https://youtu.be/PYyiYuEgUKI

Env

• Matrix version: [Server] INFO Matrix version 7.18.4
• Server version: [Server] INFO This server is running Paper version 1.21.11-100-main@4873e3f (2026-01-29T11:02:49Z) (Implementing API version 1.21.11-R0.1-SNAPSHOT)