[ctrlz]

Originally Posted by NewCamaroGuy2018

I can only put in about 2 quarts of gas before it backs up to the filler door. It takes about 2 minutes to drain down to the tank.

I think there are two separate issues being discussed here.
OP has the more serious problem, which sounds like a clogged vent.
The other "incomplete fill" issue sounds like overly sensitive automatic shutoffs at the fuel vendor.

[CalgaryZL1]

I’m pretty sure there is a anti-siphon float to prevent someone from sticking a hose down to steal gas and accidental rollover, with vehicles this new and it being a problem I’m starting to think it has something to do with that

[ctrlz]

I don't think there is any system to prevent siphoning. There is a check valve in the fill tube to prevent spit back.
The rollover spill prevention is handled by the fill limit valve, also internal to the tank. This closes the tank vent when liquid fuel fills the space and prevents liquid from entering evap lines.
During normal filling operation liquid goes down the fill tube to a point where it narrows forming a liquid seal. Air and fuel vapors exit the top of the tank through the tank's internal evap vent line. That line runs over the right rear wheel to the charcoal canister, but there is also a Y connection back to the upper fuel fill pipe. This is where the air/vapor goes while filling. At the top of the pipe that air either exits to atmosphere or can be picked up by the vapor recovery system of the pump hose.

[2023ZL1RapidBlue]

I had this issue on another vehicle I had. Turned out to be the Canister Evap solenoid sticking shut. As many miles as this beater pig had, the evap line wasn't clogged.