How far have you driven since doing the forced regeneration?
Also, have you tried resetting the fuel trip computer, and driving 10 miles or so to see what it says. If you've not done many miles since, it could be as simple as starting the forced regeneration reset the trip computer, and at the moment it's including the fuel used in that regeneration (where it will have used quite a bit of fuel and travelled zero miles) in the current calculation.
It would also be worth seeing how far the car travels on a tank of fuel to get a real MPG measurement rather than just what the trip computer says. It's important to know whether the car is really using lots of fuel, or just telling you that it is....
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