Slime block piston elevator intermittently fails to launch players upward in Paper 26.3 #14344

Expected behavior

When a piston pushes a slime block upward while a player is standing on it, the player should be launched upward consistently.

Observed/Actual behavior

On Paper 26.3, the slime block piston elevator works inconsistently.

Sometimes the player is launched upward normally, but sometimes the exact same elevator fails to launch the player.

No changes are made to the contraption between successful and failed attempts.

When the issue occurs:

The piston activates normally.
The slime block moves normally.
The player remains in contact with the moving slime block.
However, the expected upward launch velocity is not applied.

After several attempts, it may start working again, and later fail again.

This appears to be an intermittent physics/velocity issue rather than a completely broken slime block piston interaction.

Steps/models to reproduce

1.Create a simple vertical slime block piston elevator.
2.Place the player directly on top of the slime block.
3.Activate the piston so that the slime block is pushed upward.
4.Repeat the activation several times without changing the contraption or player position.
5.Observe that the player is sometimes launched upward normally, but other times the player is not launched even though the piston and slime block move normally.

The issue is intermittent and may require multiple attempts to reproduce.

Reference video showing the expected slime block piston elevator behavior:
https://packaged-media.redd.it/u3dpb8164ksh1/pb/m2-res_720p.mp4?m=DASHPlaylist.mpd&var=sgpssan&v=1&e=1791093600&s=826b92ca9a0ca43619dcd1fdbfbe2e19aac3c7cd

** This is not my video, but it demonstrates the same behavior.

Plugin and Datapack List

Paper Plugins (1):

• Timberella
  Bukkit Plugins (4):
• DiscordSRV
• LuckPerms
• WorldEdit
• WorldGuard

Paper version

[09:56:11 INFO]: This server is running Paper version 26.3-147-main@ddb45eb (2026-10-03T19:24:54Z) (Implementing API version 26.3.build.147-beta)
[09:56:11 INFO]: You are running the latest version
[09:56:11 INFO]: Previous version: 26.3-145-6bc56ab (MC: 26.3)

Other

This appears related to the previously reported issue:

#14327 - Slime block piston elevator no longer launches players upward in Paper 26.3

#14327

That issue has been closed, but the problem is still reproducible on the latest Paper 26.3 build.

The main difference is that on the current build, the behavior is intermittent: the same elevator can work correctly and then fail again without any changes.

https://spark.lucko.me/Cmc8MfE0O8