Player Heads in advancement icons render incorrectly since Paper 1.20.6+

Since 1.20.6, player heads in advancement icons no longer display the profile skins correctly to the client, without the server running minecraft:reload twice. This happens regardless of whether the server is in online mode, or behind an online proxy.

Expected behavior

Expected behaviour is that upon a server starting or restarting, advancements with player heads for an icon would display correctly for any clients joining, without needing commands being run to reload them.

Observed/Actual behavior

Advancements with player heads render with the default player skins, until minecraft:reload is run twice by a server operator.

Steps/models to reproduce

1.20.4 - Working Correctly

• Create a 1.20.4 server
• Install the slabservers3-main-1204 datapack
• Start the server, and grant all advancements
• Observe that the playerhead works correctly for the "Etho's Head" advancement

1.21.1 / 1.20.6+ - Not Working Correctly

• Install the slabservers4-main-1206 datapack
• Start the server, and grant all advancements
• Observe that the playerhead does not work correctly for the "Getting Started" advancement
• Run the minecraft:reload command twice
• Observe that the "Getting Started" advancement now renders the player head correctly.

slabservers3-main-1204.zip
slabservers4-main-1206.zip

Plugin and Datapack List

Plugins:

N/A

Datapacks:

• slabservers3-main on 1.20.4
• slabservers4-main on 1.20.6+

Paper version

1.21.1:

[16:39:42] [Thread-8/INFO]: This server is running Paper version 1.21.1-17-master@fb53074 (2024-08-16T14:15:24Z) (Implementing API version 1.21.1-R0.1-SNAPSHOT)
You are running the latest version

1.20.4:

[16:37:33] [Thread-8/INFO]: This server is running Paper version git-Paper-497 (MC: 1.20.4) (Implementing API version 1.20.4-R0.1-SNAPSHOT) (Git: d8d54d9 on ver/1.20.4)
You are running the latest version

1.20.6:

[16:34:48] [Thread-8/INFO]: This server is running Paper version 1.20.6-148-ver/1.20.6@20f5165 (2024-07-02T15:37:33Z) (Implementing API version 1.20.6-R0.1-SNAPSHOT)
You are running the latest version

N.B. Each of these servers was dynamically provisioned for testing purposes, as part of debugging the issue.

Other

Relevant threads from Discord: #1, #2

### Tasks

Start by reproducing the issue with the supplied 1.20.4 and 1.20.6+/1.21.1 datapacks, granting advancements after startup and then trying minecraft:reload twice. Trace the advancement player-head loading path during server startup and reload; done means the correct profile skins render for joining clients without commands.