Passengers added to a player with `Entity#addPassenger(Entity passenger)` don't remain attached

[eccentricdevotion]

Expected behavior

Entity passengers riding a player should move with the player.

Observed behaviour on Spigot:

spigot_passengers.mp4

Observed/Actual behavior

Entities are not attached to the player and do not ride them at all, if the player moves the entity is left behind

Observer behaviour on Paper:

paper_passengers.mp4

Steps/models to reproduce

Basic code to reproduce:

Bee bee = (Bee) player.getWorld().spawnEntity(player.getLocation(), EntityType.BEE);
bee.setAI(false);
player.addPassenger(bee);

Plugin and Datapack List

[22:26:59 INFO]: ℹ Server Plugins (2):
[22:26:59 INFO]: Bukkit Plugins:
[22:26:59 INFO]: - LuckPerms, TARDIS

[22:27:26 INFO]: There are 4 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [file/tardis (world)], [paper (built-in)]

Paper version

version
[22:20:54 INFO]: Checking version, please wait...
[22:20:55 INFO]: This server is running Paper version 1.21.5-28-main@55f2020 (2025-04-22T20:12:27Z) (Implementing API version 1.21.5-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.5-27-df42993 (MC: 1.21.5)

Other

TARDIS plugin on GitHub
Command used in videos - https://github.com/eccentricdevotion/TARDIS/blob/master/Core/src/main/java/me/eccentric_nz/TARDIS/commands/dev/TARDISDisplayItemCommand.java#L98