I know this was a known problem that I think was said to be fixed, but isn't fixed. I was just playing a coin drop game in Hammamitsu and won 50 fuel. My ship had full boost fuel before the game, as I hadn't used any. After I "won" the fuel I only had 50. It cost 2784 gold to refill that fuel back to a normal of 514. This is with a level 54 Royal Navy Galleon.