I remember the 1st time I filled my rig (4 weeks ago).
I couldn't get the fuel to go in at first without spewing all over. Then, when it was almost full, it shot out all over me, the rig, and the ground and never shut off .. I'm one of those people that stays right there when filling up -- like the pretty sign shows on the side of the fuel pump .. LOL
I did some digging and saw the 2 flaps .. and an article from FCA about after the 2nd fill up, if it still happens, then replace the filler neck .. I would guess that the flaps are so "tight" initially, that it takes a couple times of use to break them in.
-- There is a white filler assist under the pass seat for adding anything (even fuel from a can). I've never run additives on anything by my Cobra (and that was only Octane Boost when I couldn't get the fuel I needed).
Now you all have me wondering .. about the seal on the door, etc. I'll have to dig into the manual and get some parts-breakdowns and see what it looks like (and open my door and see if there is dust build up).
Have a great weekend!