So if you lose a fief you also lose any profit from selling goods when you had it? (assuming the trader sells after you lose it)
That seems like bad mechanic.
If you're a fief owner, and someone sells goods they bought at your fief somewhere else, you get tax income. Likewise, if someone sells at your fief, you get tax income.
If someone takes that fief, they are now the fief owner. So, uh...tax income goes to him. You know, because he owns it.
I'm very curious why you don't like this- do you feel that a previous owner of a fief should get a certain grace period in which he automatically retains tax income from a fief currently owned by a new owner? The silver would have to go directly into the previous owner's inventory since he wouldn't own the fief anymore, as well.
It isn't as if all the silver you've gained from tax income while owning a fief is transferred to the new owner when they take it or anything. You just don't get any more tax because....you don't own the shit.