[Patchman123]

I think the reason this wasnt modelled in the game originally is probably because having one tank empty and one tank full would mess with the flight model of the plane. i.e. if your right wingtank has less fuel than the left, your left wing will want to dip.

So the game gets around this problem by using fuel from all tanks at the same time, regardless of where the leak is.

Would be interesting to see if any modders can fix this but it could mean a lot of work

Yes, it would be great. All you have to do to compensate for this problem is add some aileron trim to try and compensate for this. It's no big problem, really.