In Shadowlands seasons 3 and 4, you could unlock successive levels of tier set appearances if the tier piece was pulled from the Vault or catalyzed from an M+ drop and then upgraded using valor. In the case of the Mythic raid set, you had to pull the piece from the Vault at max level natively but the possibility still existed.
So far in Dragonflight, none of these rules are applying. I’ve pulled two 415 tier pieces from my M+ Vault slots so far but my appearances tab shows that I have collected zero pieces from either the Heroic or Mythic raid sets.
There is a thread on the PTR forum about this below but I’m putting it here because it is currently a bug on live retail if we go off the Shadowlands behavior. Tier Appearances Not Unlocking On Upgrade?