To recreate:
Fill a standard furnace with raw materials and fuel.
When the flame graphic is low, stop the server.
Restart the server and open the furnace GUI.
The flame is high, and you can smelt more than was initially left.
To temporarily fix, open the in-game menu and return.
Thank you for your report!
We're actually already tracking this issue in MC-10025, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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