The problem

The game only performs at most 65536 scheduled block updates (TileTicks) or liquid updates (LiquidTicks) (since 1.13) per tick. This number seems to be arbitrary. Any additional scheduled updates are performed in the next tick. This could cause contraptions to malfunction if they rely on the update being already performed.

In case performance was the reason why this limit was added, you likely won't care much about performance if you have 65536 scheduled updates since this prevents the server from responding for several minutes anyways.

The affected method is net.minecraft.world.ServerTickList.tick() (1.13.1 decompiled using MCPConfig).

Video examples

• Block updates: https://www.youtube.com/watch?v=1GWaZ5vEd3A
• Liquid updates: https://www.youtube.com/watch?v=PAMe-YoVEFM

How to reproduce

1. Download the attached world MC-123728 test world.zip
2. Open the world and press the button next to the sign saying "Activate command blocks"
   This activates 65540 command blocks
3. Write something in chat and wait until it appears to make sure the server caught up
   → "One tick later" will appear 4 times in chat because 65540 block updates were scheduled