Interesting. The way I read the Fuel Systems/Fuel System Part #'s/
Fuel Tank Selector Valves page, your truck should have a 3-port valve and a mechanical fuel pump. If that is the case then the only way the transfer can be happening is if the valve is bad and there is either siphoning or at least a transfer due to gravity going on.
So the first thing to do is to ascertain what kind of fuel system you have. Does it have a mechanical pump on the engine and a 3-port valve, meaning there are only 3 lines to the valve on the frame under the driver's seat?
I ask because the normal way this happens is with the notorious 6-port valve, which is known to return fuel to the other tank from which fuel is being drawn. (As Ray Cecil about that.) But if you have a 3-port valve then there's no return.
Gary, AKA "Gary fellow":
Profile
Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI