And I can even add what's the problem: For testing I used the same LBS aircraft and I chose the maximum amount of fuel when i was asked to. It did fuel this time, but the info text showed kilograms. I suppose it did not fuel the first time because it took my starting fuel as KGS (like it was 7000 LBS but it thought it was KG), but did take the fuel I entered (10000 lbs) and recalculated it into KGs (around 5000 kg) and then thought - "ah no, that's less fuel then there already is so I won't do the fueling".
So tldr: It does not understand that the starting fuel value is also in lbs and has to be recalculated into KGS before it can compare it to the new demanded fuel value I enter later.