If the tank has conventional clunk venting with the vent stuffed at the top of the tank, when its inverted it'll act like a uniflow when that one vent is covered with fuel -- IOW, the engine has to suck harder because an equal amount of air needs to get sucked in through the vent to match the fuel going to the engine and has to overcome some depth of fuel to do so. This would naturally cause the engine to go lean compared to upright when the vent is not immersed in fuel freely allowing air in the tank

That it helps to richen it up leads me to believe this is the situation you have.