Ola....
Had the same play a few years ago; and your Linesman is correct.
If I understand your play correctly, the receiver's first contact after possessing the ball while in the air was with the pylon - which, by rule, and if properly placed, is out of bounds behind the goal line. Therefore, the receiver has not completed the catch by returning to the ground in bounds (Rule 2-4-1).
And, yes, the coach of the offense - who happened to be on my sideline - was not too thrilled.