Hello, I just cast about 300 hydras, and zero familiars spawned.
I think having familiar on your action bar allows you to have up to six familiars in play. Without that, my enchantment wasn’t proccing either.
On the plus side, my casual Hydra spam build somehow generates attunement in undercity while I’m still in the time-stop zone, and also generates masterworking materials in Nightmare Dungeons after the boss is already dead. (or before walking away from the nightmare dungeon entrance, for that matter)
As of the recent update, I am no longer receiving free mats in nightmare dungeons, but haven’t tested undercity behavior. Also fixed was the original poster’s reported bug. My Hydra spam now correctly summons Familiars via the enchantment w/o needing to also have it on the action bar.