Fuel filter Orientation makes a difference?

[Stanley]

After a drive, checked the clear filter before the mech pump while the car was idleing - only about 1/8 or less full. Looked at it carefully and could see bubbles appearantly being drawn into the fuel pump. It's installed so it sits level, horizontally. Tried tilting the filter (due to the hoses I could only tilt it about 45 deg both ways). When I tilted it the filter started filling up. Within about 45 seconds it was filled up about 5/8 to 3/4 full and no bubbles. Put it level again. Slowly (took about 3 minutes) the level went down to almost empty. Repeated the test 3 times - same result every time. Doesn't seem to matter which end is tilted up. WTF?

Have been having some fuel problems related to a combination of things - mostly sorted out now. (Took a 20 mile freeway run a couple days ago, no problem til I came off and stopped at some lights. When I hit the gas it sputtered til I pulled the choke lever for a sec. Couldn't repeat the problem later or today - seems unrelated to the filter thing - will check the float valves and levels).

Concerned it might cause fuel problem and/or might hurt the pump if it's sucking air. Tank is clean, vents & fuel lines clear, filter elements clean, in/out ends correct. Took off the gas cap so no venting issue. Only running mech pump for now, another filter by the tank. Plan on installing it vertically, I guess, but puzzled by this "strange behavior".