I was doing some timewalking with my friend 2 weeks ago, for Wrath timewalking. He had recently purchased TWW and was leveling a new character. I had not yet bought it and was stuck at 70. I was getting credit for the great vault, but he wasn’t. Timewalking dungeons are heroics, so they normally provide slots for the vault at 80, but since I didn’t have TWW, I was Dragonflight level capped and still got credit. We both recently got to 80 and I had a vault reward waiting for me in Dornogal when I level capped, but he didn’t.
I assume this is some kind of coding bug where it mixes up the difference between level cap in previous expansions and level cap in the current one, versus dishing out rewards from the vault. Not really a high priority bug as this is a very specific case, but I found it interesting and thought I’d mention it.