From the Terraria Wiki:
The first three Demon Altars destroyed will trigger the creation of
three new Ores, in the following order:
The first destroyed Demon Altar or Crimson Altar will trigger the
creation of Cobalt Ore or Palladium Ore. The second destroyed Demon
Altar or Crimson Altar will trigger the creation of Mythril Ore or
Orichalcum Ore. The third destroyed Demon Altar or Crimson Altar will
trigger the creation of Adamantite Ore or Titanium Ore.
Did you destroy at least 3? It also says that Hallow and Crimson/Corruption should spawn upon its death, not the altars. If that's not the case, i would assume it is a bug yes.
Else see this post from someone with a similar issue on reddit.