It was a recent box wasn't it @Grrrrrr I reckon the shaft could be worn. Was it a recon or brand new, maybe the shaft is just out of tolerance?
Refurbed one from Ashcrofts 2 years ago. Never considered the shaft wearing around the seal. Guess it is possible. The seal I took out a few months back had no spring on the back so that one was definitely fitted incorrectly. The replacement seal I put on was Corteco. I figured I must have messed something up like maybe not quite getting the seal square in the hole. IIRC it was 3/8" below the lip.