Sometimes when the game starts, the advancements will just not load. I don't fully understand what happens but I'll document all my finding here if someone else wants to take a look at this.

• The advancements seem to be missing because a packet sent on the server is not received on the client.
• The problem seems to be the client processing the received packet
• Rejoining the world or /reload does not fix it although the packet is sent again. However restarting the game does fix it.
• When the client receives the packet forge posts an event of type NetworkEvent.ServerCustomPayloadEvent
• It fails with an ArrayIndexOutOfBoundException: null in EventBus#post / Event#getListenerList

Maybe the problem lies somewhere else but I could not figure out where.

Should be fixed when updating to 1.17