I just encountered this issue today. I'm on my second Cmax, my first was a 2013 Cmax SEL, that I had ZERO problems with but sold it at 135K miles and got a 2018 SE which is having this damned fuel door not opening issue. I have not contacted a dealer yet (should still be under warranty) but I did look for a manual release in the back and couldn't find one. I have a boot with a full sized spare, fuel can, jump box etc in it so it's not exactly wide open to see where a manual release might be, but I sure didn't see one where I expected it to be. What I am not geting is what fuel tank press has to do with the fuel door opening? Sounds like there is no common fix here and THAT worries me.