That might indicate that something is off center.Yes the bullet is only expanding where it is seated, but bulge on one side of the neck.
Try seating the bullet initially about 1/4 inch just to get it started, then rotate the case in the shell holder and seat the bullet a little more, then repeat the process until seated to the desired COL.
If that doesn't work, try full sizing a dummy case with a standard F/L die and a floating expander which should help align the neck with the case body then seat a bullet per above and see if that helps.
Does the shell holder have any accumulated debris in the rim area which might tilt the case off center? Also, check the fit of the shell holder in the ram to verify that it is centered.
Does the sizing die have any accumulated lube? How often to you clean the sizing die. I dry brush mine out after each loading session. Also, some tumble media residue can accumulate in the shell holder and dies which can cause problems.