Basically, your jump is cancelled unexpectedly in this specific case.
How to reproduce this bug:
Create a superflat world and allow commands (superflat so the terrain is convenient);
Run the following commands:
/setblock 47 71 73 prismarine_slab
/setblock 47 72 75 prismarine
/setblock 47 71 75 prismarine
/tp @s 47.063848975254324 71.5 74.15667647020095 -2.2 50
Then, perform the following input, aka "rex bwmm with d" (it is not that easy):
Jump with S, walk 1t with S upon landing, jump forward with W+D and immediately release D, hold space.
The video shows how to reproduce this bug. (Sorry for the mkv format if that is inconvenient)
This bug can be consistently reproduced as long as you do the "rex bwmm with d" correctly.
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I found that the player tries to "step" onto the prismarine block.
An easy reproduce method:
Create a normal superflat world.
/setblock 30 -60 30 birch_slab
/fill 31 -60 31 31 -59 32 birch_planks
Then
/tp @s 30.7 -59.5 30.65 -30 50
(X should between 30.699999889 and 30.700000088, Yaw should between -0.01 and -44.99)
Then run and jump at the same time, you will get a Jump Cancel (because you are trying to step onto the birch planks). and if you run 1 gametick (0.05s) then jump, you can jump onto the birch planks, which is 1.5 blocks high.
This doesn't only affect stepping up into blocks, you can also just run/walk into blocks completely for 1 tick.
(the jump shown in the video is not possible without the bug — the picture shows the point where the players coordinate is inside the block and still on the floor. For this to work there has to be a gap at the bottom of the wall)
[media][media]
Please attach a video of this happening in a vanilla environment. Your version of the game appears to be modded.