You may be surprised to know that some staff at Greater Anglia are confused as well. In terms of a Stratford to Shenfield journey you’re probably better off using the Elizabeth line as it’s then free all the way. If you want to use the fast trains then you need a boundary zone 6 to Shenfield ticket which can be bought online as Colin suggested in the post above. The 60+ Oyster is a zonal ticket, so there is no need for the train to stop at the last station where you switch to the boundary ticket.