Well, the Swedish government (and public) have a particular liking for high ethanole content fuels - they are the most ethanole-infested country in the EU. It's a real disease up there!
It wouldn't surprise me if this damages the tank insides in the long run - I already have plenty of problems with french regular petrol (usually E10) - After a few thousand kms there, I had to re-drill the tank mounting brackets to make the tank fit into the bike again! If the vent line is the same material than the tank, it sure can get damaged / changed in shape over time...
However, I would thoroughly check the inside, and remove the pump unit for a closer inspection.
The fuel leaks out from the drainpipes from the tank so yes - I'm pretty sure it's the pipes inside the tank that leaks. I have done som experimenting and it seems like the leakage is on the top part of the piping. If you keep the fuel level at about half the tank it seems fine.
Some early tanks had a replaceable piece of hose inside later they were moulded. Was on a ride with a friend with an early RSV and he started to lose fuel out of the pipe after a fill up if he kept the level down it was OK.
We pulled the fuel pump so we could get inside and replaced the fuel rubber hose that was there with a new piece and it was OK from then.
Look inside the tank with a spotlight and see what colour the pipes from the top to the bottom are, mine are white in the moulded tank.
no matter the brand of bike, the wind is the same.
Remind me to never take my bike to Sweden. It seems they put something in the water that makes the birds quit singing.