Same issue here - started a co-op career in Bahrain with a friend last night. Did all the practice sessions to 100% R&D completion. Short qualifying and 25% race. Added an extra lap of fuel for race strategy. After 3 or 4 laps we both had huge fuel consumption issues and barely made it to the finish lapping 6/7 seconds off the pace.
We then put 3 laps of extra fuel in at Spain and more than half the surplus was gone by lap 7?
This only seems to affect this mode, I haven't had any issues on My Team, Braking Point, or GP at all, so does seem to be an unintended bug.