Had the same issue with my 2018 JLU. After seeing my 2020 JT gas tank filling as expected, I reached out to my dealer and they replaced the JLU tank a few months ago under warranty. I'm not driving much lately, but so far it looks like a 2 gallon increase.I'm getting tired of only being able to put 16 to 17 gallons in my 20+ gallon tank...
Yes I tried to get it fixed when new...
No the dealership was not helpful at all
I figured I could live with it
But seems to be getting worse with time
Would love to know the reason for this, my guess is something to do with the vent