Demo Warlock, Power Siphon issues

On one of my warlocks (Lockstock - Alexstrasza, Level 35), Power Siphon is never usable. However, it is usable on another warlock, (Drdotsalot - Black Dragonflight, Level 110)

I’ve seen this reported on the WoW sub-reddit and on the WoW forums months ago but either I am missing something or this is still bugged

Scenario:

Spec as demonology, choose Power Siphon talent. Toss spell in bar, or don’t.

Cast Hand of Gul’dan with 3 Soul Shards, returning 3 Wild Imps. Try casting Power Siphon, see error text on screen “You can’t do that yet”.

Tried:

  1. Having more and fewer Imps out (1-8)
  2. Waiting longer to cast Power Siphon - from the moment the Imps spawn to after the mob is already dead and I have even more imps available
  3. Disabled all add-ons, reloadui, relog, etc
  4. Cast using: hotkey, click spell in spell book, /cast Power Siphon
  5. Spec different talent and spec back
  6. Change specs, change back

Again, if I follow the same steps on a 110 warlock, it works first try. Why doesn’t it work on my level 35?

----------------EDIT-----------------

Two things happened in relatively quick succession so I’m not sure what triggered and caused the spell to start working but:

I was killed via PvP, revived, killed one mob, leveling me to 36. I looked at my bars and noticed Power Siphon was activated. I was then able to cast it. It continued to work on subsequent mobs.

¯_(ツ)_/¯

Had the same problem. Sarai-Norgannon; Power Siphon was unusable levels 34 and 35. After reaching level 36, Power Siphon immediately became usable.