I just tried Stage 12 of the Void Keep seven times and got the same results: lessers and superiors but no greater potions.

Honestly just sounds like RNG. You can contact Support if you'd like, but it does just sound like standard drop rate stuff.

I'd normally be 100% on board with that, but I used a LOT of energy refills. I didn't keep track of how many TBH, I just kept spamming them as needed, but we're talking about hours of this (I used auto-attack and set a phone timer every time so I'd be sure to check back when refill time came). Can anyone show a current screenshot of themselves getting a greater void potion from stage 15? Someone in my clan said that you only get greater potions at the max stage but if that's the case, why is the mission even implying that you can get something from Stage 7+? Hell, why am I getting SUPERIOR potions but not greater potions? You'd think the odds would be swapped. That might be the issue. Maybe a dev accidentally has Superior Potions set to the Greater Potion's average pull chance and vice versa.

Yeah, I understand the sentiment. 

But again, it's nearly 100% just a situation of RNG. You are 100% invited to contact support if you'd like them to look into it though. 

I thought this was support? lol

It is, but since there's: 

A) no known bug with option keeps

B) no confirmed/valid complaints yet with data backing up an issue

the best play if you are still skeptical would be to contact customer support directly since they are able to check your game/data whereas this section of the forum is frequented by moderators - who are volunteers that are unable to directly affect your account.

Yeah, I'm not sure what data I could submit. I've hit the dungeon on and off since I last posted and still no greaters. Ah well, I'll contact CS, thanks.

No worries. 

If something is amiss - whether on your side of Plarium's then support would have a much better idea.