The bug

You can only right click a furnace minecart 8 times with fuel, after that you have to wait until enough fuel is used up. But if you reload the world you can right click 8 times again client-side. Once you update the coal item stack you used the count is corrected.

How to reproduce

1. Place a furnace minecart
2. Switch to Survival or Adventure mode
3. Right click the furnace with a stack of more than 16 coal until you cannot right click it anymore
4. Reload the world
5. Try to right click the furnace minecart with the coal again
   → You will see that coal is used but once you update the coal item stack the count is reset

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

This happens because the server is not sending the value of the fuel tag to the client.