All well broken in smooth operating pump guns SHOULD do this.
I have three Mossies.. a 500 12Ga. (NIB Apr. 1990), a 500 20Ga. (NIB Jun. 2006), and an 835 (NIB Dec. 1993). The 500 12Ga. has well over 20k documented rounds through it; it must not be broken in yet as NONE
of my guns do as described. My brothers Maverick bought in 1992 does not do that, neither do any of my friends 500's or 870's nor any of the ones I have shot over the past 25+ years.
Slamming the bolt to the rear and ejecting the shell suggests to me that the OP has it looked at by someone in the know. His chosen course of action is not only wise but commendable IMHO because if there really is a problem as some of use believe, the next generation of smiths will have an example to diagnose and correct in an expert setting.