At first I thought it was as simple as my maximum fuel only being 120 instead of 124 like you say, but it's not just that. The more important issue I'm having is that when my fuel goes over the maximum, either from daily login reward giving fuel or from refueling while still having some fuel, then any fuel that goes over the maximum is lost. Prior to getting level 100 that fuel wasn't lost under the same circumstances.
For example, this morning I got the daily login bonus of 15 fuel, and I was at maximum fuel, so it said I had 139 fuel. After doing one normal Bigboy run I then Had 111 fuel. If it was just that my maximum fuel is 120 instead of 124, then I should have had 125 fuel after doing one Bigboy run on normal.(120 max + 15 for the login bonus - 10 for Bigboy).
The same thing happened when I paid 20 token to refuel when I was at 2 fuel. It took me to 126 fuel, then I did one normal Bigboy run and afterwards I was at 111 fuel. In both instances the fuel that exceeded my maximum was lost.