[MCM-1087] Behavior pack animation controllers break if a passenger is added on the entity's spawn event

Partner Rank:

Partner Team Reporting:

Pathway Studios

Verification builds and Platforms:

Minecraft-Release_UAP_x64_4211827 - 1.16.0.2 RC1

Summary:

In the provided world players cannot place a bun onto the tray until they reload the world, apparently due to the animation controller not running.

Impact:

Breaks a core mechanic of the game and makes behavior pack animation controllers unreliable.

Repro Steps:

Open the world provided in Slack and attach the provided RP and BP
Summon a new tray with:
```
/summon custom:tray
```

Right click on the tray with the Burger Bun (Bottom); nothing will happen. If you open the mob debug panel and select the tray you'll see that it has the component group "ms:ingredient6".
Close and reopen the world
The bun now appears on the tray and you can place cheese on top of the bun.

Observed Results:

The bun does not appear on the tray until the player relogs.

Expected Results:

The bun instantly appears on the tray and the player is able to place cheese on top of it.

Screenshot/Video?

Yes, attached.

Regression Builds:

Minecraft-Release_UAP_x64_4011639 - 1.14.60.5 RC8

Notes:

The component group "ms:ingredient6" sets the entity's variant. The behavior pack animation controller "controller.animation.tray.update" in the file "tray.animation_controller.json" should transition to another state when the variant is changed; however, it does not appear to do so. This implies that the animation controller is not running.

After further testing it appears that if you spawn an entity and the spawn event for that entity adds a component group that adds a rider (via "minecraft:addrider" component) the behavior pack animation controller on the base entity don't start running. I lowered the priority of this bug since it can be worked around by adding a 1 tick delay before adding the addrider component.