If the server sends an incomplete ClientboundPlayerInfoPacket to the client, further down the line the client will call YggdrasilMinecraftSessionService::fillGameProfile to fill in the blanks. Problem is, that method involves making API calls, and it uses UUIDTypeAdapter::fromUUID to get the UUID string in order to make said API calls. The API expects a URL where the UUID is lowercase, but UUIDTypeAdapter::fromUUID returns the UUID as uppercase, causing an IllegalArgumentException when it tries to create a new GameProfile object as the API response would be invalid.
 
The simple fix would be to turn the URL fully lowercase before making the API calls.
 
Now, that's a very niche bug that never really happens in practice unless the server or client happen to be modded (which is how I found this out in the first place), but since it's technically still a bug with Minecraft's code which renders some methods broken, I thought I'd report it nonetheless.