I don't see this behaviour at all.
Just round up what simbrief gives you. If your minimum takeoff fuel is 85660, take 85800. Make sure you enter all the correct values for alternate and reserve fuel as calculated in the OFP and don't forget to import the winds. If the flight computer still predicts you have not enough, be sure you have entered the planned stepclimbs.
And don't expect a perfect calculation from the computer. The computational logic of the fuel page is always a bit overcautious while you are still on the ground.
At least it is this way in the narrowbodies, like the A320.