Initially, the reports we received suggested that changing tyres on the grid in a wet race would trigger the problem. As we received more information it has become evident that simply joining the race is enough for the fuel load to be reset to the maximum value that was allowed before the most recent update (120 litres).
For 100% races, 120 litres is insufficient fuel to complete the full race distance. I don't believe that lower race distances are affected but potentially 75% leagues in Rookie tier with very low FE may also require more than 120 litres.
Until this fix is rolled out the only potential workaround is to wait until after the race has started before joining, by which time the cars will already have started the race with the fuel load you specified in the pre-race strategy.
I would like to point out that nobody has confirmed this workaround works.
CEO & CTO