B2 custom entry stamp missed the expiration date

My parents arrived last Tuesday, but we just found that their custom entry stamps missed the expiration date, only with a mark "B2".  Compared with the stamps last year, where there was expiration dates with period 180 days from their entry date, everything else looked the same, why only the expiratio date was missed? 

Do we have to go back to the airport (how can we be allowed to go the custom office?) to ask for the expiration date specified?  Or because I-94 is now electronic, with this marked or not, does not matter?

Any experience or suggestions?  Thanks!