I can’t remember which post I explained it on, but leaving it on BOTH allows the fuel to crossfeed when parked. If the right wing is left higher than the left and you have a certain fuel configuration (including during refueling) it can siphon to the left wing, where the fuel vent is, and spill overboard.
So when parked, LEFT or RIGHT is correct. During flight, you use BOTH unless it’s been feeding primarily from one or the other (this can happen) and is becoming unbalanced. Or maybe you had uneven fuel to begin with.
Where the sim messes up is that in a “ready to fly” or “ready to start” state - it should have been set to BOTH by that point.