Is that working as intended or is it a fix that should be applied.
I had a further testing of the hirelings and found some interesting outcomes:
1. Not all Permanent Hirelings are impacted by this. Teal (Dino) is the most notable one, not accepting the Zombification and will be damaged from the Negative damage. But the Scarecrow and Shield Guardian (both counts as Iron Defenders) can be Zombified and can be healed via Negative Energy (rather than harmed like Teal), as can Elieri (Level 3 Cleric), the Feywild Muse, Froggo, are all able to be Zombified and heal from Negative Energy, whilst the Panther and Lioness both REJECT Zombification but CAN be healed by Negative Energy - suggesting it's not a direct Race issue and could be a Permanent Hireling difference issue (Assumign Teal is supposed to be healable via Negative Energy as well whilst it currently does not).
2. Whilst the few Plat Hirelings I tested out with did not accept the Zombification outright (but I'll need to go through more of them to really be sure, but it looks like it doesn't impact on them unless if they are a Barbarian Hireling). Suggesting that this might be a Permanent vs Plat/Gold Seal difference, and that some of the Permanent Hirelings not being able to healed by Negative Energy on being Animated (or from Death Salve via Alchemist) I suspect is likely a bug.
I suspect from that testing, that it might be a "design feature" that normal Hirelings are not able to be Animated and receive healing from Negative Energy. However, this is only something the Devs can say for sure and if they feel that it (normal Hirelings can heal via Negative Energy after being Animated) should be updated to do allow them to do so, they will probably get on it further down the line given it's apparent low impact status (very rare instances of it happening).
As for Teal being the sole Permanent Hireling not able to receive healing from Negative Energy after being animated whilst every other Permanent Hireling can heal, given this recent testing, it's probably a bug and likely will receive a low priority fix when there is time. (more likely fix than the change to all normal Hirelings)
J1NG