There are 2 connections to the canister. 1 is the purge line that goes forward to purge valve connected to the engine and should not have any effect on filling.
The second is the vent valve / line and somewhere in the valve or the line connecting back to the tank (or in the filler neck) is likely where your problem is. When filling the tank. the air displaced from the tank needs to flow freely to the canister and then out, anything preventing that will cause what you are experiencing.
I'm going to be back with my 2017 Expy next Wednesday, I can look more closely at the routing at that time.