The module around pump is designed to let car run with as little as two cups of fuel left in tank to prevent pump running dry at least on earlier models, I assume same on all vehicles now since they went to modules to stop pump warranty issues from tanks running low and starving pump. Pump can die with as little as 30-60 seconds of dry running and why all owner manuals now say to never let car run out of gas.
There is a small one way valve that allows fuel to come in module on demand but not seep back out, if it has trash stuck in it or not sealing fully then could easily be the issue there.
One gallon was not enough on mine to prime pump once it ran dry on a slight slope at indicated 1/8 tank, when doing pump work I always use a minimum of 3 gallons back in before i turn pump back on. It's stupid to burn up a brand new pump, I saw it plenty of times though at the parts store. When questioned hard most customers with the issue usually admitted they only used like 1/2 gallon thinking that would be enough. Not nearly, you want to really flood that thing.