This article contains spoilers for Stranger Things season 4 Volume 1.
Stranger Things season 4 saw Hopper face a Demogorgon in a Russian prison, an apparent plot hole that may actually hint at major changes in the Upside Down. Stranger Things has always existed against the backdrop of the Cold War; the experiments at Hawkins Lab are actually inspired by the CIA's sinister Project MkUltra, as the U.S. attempted to unlock the potential of the human mind as a weapon against the U.S.S.R. The Cold War finally came to Hawkins in Stranger Things season 3, when the Russians created a Gate of their own into the Upside-Down. Although the Gate was destroyed, Hopper was captured and believed killed by his friends and family.
Stranger Things season 4 picked up from this point, with Hopper learning the Russians had captured a Demogorgon before the Gate was closed. They were now using it in cruel sport, pitting desperate prisoners against it to keep the predator alive. But, as Hopper rightly realized, the Demogorgon's presence should be impossible. All creatures in the Upside-Down are bound together in a hive mind centered on Stranger Things' Mind-Flayer, and the closing of the Gate should have killed the creature when it was cut off. At first glance, this seems like a clear plot hole.
It's possible that, unlike the Demodogs seen in season 3, Stranger Things season 4's Demogorgon was strong enough to survive being cut off from the Mind-Flayer. More recently, though, it has likely grown stronger because its connection would be restored - with new Gates torn open by Vecna. Hopper is right to believe the presence of such a healthy Demogorgon should be impossible; he correctly deduces this means Eleven isn't safe after all.
The Demogorgon's strength and general vitality is indeed troubling, and it should be seen in light of another disturbing plot in Stranger Things season 4. It's important to remember that exposure to the Upside Down was toxic for Will in season 1, and yet four of the Hawkins group were able to explore this realm with comparatively little difficulty in season 4. Vecna's Gates may well be bringing the two planes of reality closer together in some way; the dimensional shifts could explain the strange earthquakes shaking the Upside Down. If this is indeed the case, Vecna's plan could be to bring the Upside Down to Earth, superimposing it on Hawkins.
At first glance, Russia's captive Demogorgon feels like something of a plot hole, contradicting previously established lore. Hopper's own reaction to the Demogorgon suggests that isn't the case, though, but rather that the monster is tied to a developing plot point in some way. No doubt the truth will be revealed in Stranger Things season 4 part 2.