I have stood in the USA line at EWR, LAX, and SFO with no problem at the US CBP agent. At LAX, I did have to lie to the personnel manning the US residents line up and claim that I was a US resident. However the line up people are not US CBP employees and are contractors either from the airport or specific US airline (e.g. United contractors handle the LAX T7 lineup as the whole US CBP facility is utilized by UA).
I have never had a problem infront of a US CBP officer while being in the USA residents line, holding a Canadian passport.