It's how it's supposed to be. The overflow is a piece of 3/8" tygon tubing from the top of the tank toward the ground. Crawl under there and check it out.
If you keep filling it will indeed eventually puke back out the fill port as the tiny tubing can't keep up with the garden hose.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.