You can't fly this with a speed override. It is not a bug.
Fuel is mostly calculated on real planes in fuel consumed per time (hour). So if you fly slower, you flight takes longer, therefore you burn more fuel. And if you look at the performance graph and if you are on the last part of the line, then you are flying with absolutely full tanks (max. volume filled). That means, you cannot take any more fuel, therefore you cannot change your speed (flying slower burns more, flying faster burns more). So you either can fly the mission, or you cannot. It's that simple.
When you are within the horizontal part, or on the first slope, then you can trade payload against more fuel. That's why in this area you can use different speeds.
So, in the first part (horizontal line) you can take the maximum payload. The longer you want to fly, the more fuel you will need, but your limitation is the maximum take-off weight. You still have space to take more fuel.
The second part of the line, where it starts descending, you can fly further (by taking more fuel), but you have to unload some cargo and/or passengers. Neither payload nor fuel volume is at its maximum.
The last part is the most critical, where you basically have your fuel volume maxed out. And the longer you fly, the less payload you can take.
And yes, our performance system is very simplified, but it does reflect that correctly.