The canister is not blocked by the fuel itself, but by the charcoal pellets that break loose when flooded by liquid fuel.
You could be correct, but there are a lot of high-mileage cars out there that have not had the problem, so it does not seem likely to be inherent to the design.So apparently the canister wasn't designed to lay sideways but engineering decided to "make it fit"
I had the issue of slow fill from the start of having the car. Chased looking for the not missing flap, pulled the filler tube and blew out the vent before finally cutting the trunk hump to gain access to the canister.
My thoughts are it has nothing to do with overfilling the tank, that wouldn't cause what I found! The canister actually dumped the charcoal and I had to clear most of the lines and actually had charcoal floating around in the tank.. used an aquarium fish net to get that out since it had half tank of gas and didn't want to mess around inside the tank much...