That does sound likely. Air getting in and allowing the fuel to drain back to the tank overnight.
Are you using jubilee clips or proper fuel hose clamps? Jubilee clips tend to distort the hose and apply uneven pressure with the possibility of air/fuel leeks on small diameter pipes and hoses. Proper fuel hose clamps clamp with an even pressure and provide a more reliable seal.
That reminds me that I must replace the Jubilee clip on my fuel pipe as it is doing just that, letting air in and letting the fuel drain back. Takes a day or two but it is annoying when you try to start it with no fuel in the carbs.
Jubilee clip
Proper hose clips
Editor of the VVOC Magazine ReVival
"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.