Initially we thought that changing tyres triggered the bug, but over the last few days numerous reports have come in from various managers that suggests just attending a wet race is enough to reduce the fuel load to the old (pre update) cap of 120 litres.
This is insufficient fuel to complete a 100% race distance.
What is your operating system and have you updated to the latest version (for Browser "Clear Cache")?
According to the recently published changelog this bug has been patched for Windows, Android and Browser.
iGP Manager changelog
If you are using IOS the patch is not yet available.