Yes, this is bugged. It is a known bug. I do not know what causes it. It is not bugged for everyone. I have never had this issue before, no matter how fast I kill the Gelatinous Goblins.
I’ve been wanting to make a video of it, but never did.
Recently, I’ve been following the Demon Hunter Unholy Essence SSF all content gameplay thread, posted by @Beastman.
In his guide, he has a Goblin Farming video. The first two Goblins he encounters (one at ~54 seconds in and one at ~1:50 seconds in) are Gelatinous Goblins. He’s playing his non-Season Demon Hunter, Faith. He had 2726 Paragon Points at the time of the video. He has nearly all Primal items equipped. He’s playing a UE MS build. And he’s playing on T16.
He does not 1-shot kill the Goblins, but he’s killing everything pretty quick as you can see for yourself:
If anyone is experiencing issues with looting Gelatinous Goblins, my guess is that, for some reason, they’re not spawning into the game fast enough before you begin attacking.
As others have stated: When you kill a Gelatinous Sire, count “one-thousand” and attack the 2 Spawns. When you have killed the 2 Spawns, count “one-thousand” and attack the 4 Spawns.
If, after doing this, you still have issues, you may consider upgrading your computer, contacting your ISP to have them check your internet connection or both.
Here’s wishing you luck and good hunting! 
