Hateful Eidolon should draw cards when a creature enchanted by one of my aura's dies, twice in this match tonight my opponent and I noticed that Hateful Eidolon is not drawing cards if it dies at the same time as another creature.
Hateful Eidolon indeed does not draw any cards when the entire board is nuked with an Oblivion Stone in solitaire testing, and this is not correct behavior.
Both rulings we have on Hateful Eidolon point to this being incorrect.
If Hateful Eidolon dies at the same time as one or more enchanted creatures, its last ability triggers for each of those creatures. This includes Hateful Eidolon itself if it’s enchanted.(2020-01-24)
If an Aura you control is destroyed at the same time as the creature it enchants, Hateful Eidolon’s triggered ability counts that Aura to determine whether it triggers and how many cards you’ll draw.(2020-01-24)