Cost index / speed has NO effect on fuel usage whatsoever

For the route and weights below, according to Simbrief I should use about 4000kg more fuel with cost index 400 (compared to the CI 45 I did the original flight plan with). So trip fuel should be about 4-5ts more than on the screenshot. However, neither the fuel predictions nor the actual usage (!) increased during the flight with cost index 400. In other words: I was able to fly with cost index 400 and use about the same amount of fuel compared to cost index 45.

EBBR/25L ROUSY2P ROUSY DCT GTQ DCT POGOL DCT LASAT DCT MOROK DCT GILIR DCT GEVEA DCT KOGAS DCT MEDAM DCT VEVAR DCT BARSO DCT KOLON DCT RARQU DCT NESUL DCT ETOIL DCT CIRTA UA605 DJA UB730 DIR UG862 BUN DCT ASEKA DCT MUBAK DCT EGREK EGREK1B HRYR/10

I can reproduce this on every flight. The highest (!) fuel usage actually comes with cost index 0. So there's something fundamentally wrong.

Edited 10 hours ago10 hr by Fiorentoni