Bedifferent
Old man on a bike
Its a common problem on the R3T. I had a bike many years ago, don't remember which one, that had a similar problem. Helped it by drilling a few holes around the filler neck to let trapped air escape. Since you have the tank off you could try that. You just have to make sure you get all the metal filings out of the tank.
Isn't there a vent opening that is already around the filler opening? At least mine has one. Maybe that's plugged. Never had a problem.