Snickerdoodle suggested that I post here.
I currently only received 7 out of the 8 total packs available from achievements in Halo Wars and Halo Wars Definitive Edition. I should have all 8 as I have every achievement in both games.
As it is a single pack I am missing, that narrows it down to the “Playin’ the Field” achievement, but there is no indication on the packs as to which title it was from.