Can't explain with words, see attachment for video demonstration.
Bedrock is also affected, posted a separate issue for it.
Other Java versions might be affected.
100% reproduction rate.
MC-121610
Player auto-jumps in a specific block setup even when auto-jump is turned OFF
Resolved
Duplicate
1
pajicadvance
(Unassigned)
Confirmed
Minecraft 1.12.2, Minecraft 17w45a, Minecraft 17w45b, Minecraft 17w46a, Minecraft 17w48a, Minecraft 17w50a, Minecraft 18w03b
Retrieved
This is the same behavior as MC-44560/MC-8959, but this setup is pretty different from either of those. Would you say it's the same as one of them?