Something i still don’t understand from season 4.

In season 4 we see that there is a demogorgon alive in the russian prison. Since the season came out, i’ve always wondered how this is possible.

We obviously know that in season 2, when the gate was closed all of the demogorgons/demodogs died because they weren’t connected to the hive mind anymore. And in season 3, when the gate is closed the mind flayer’s flesh from dies as well, so we know that the mind flayer’s “particles” only work if a gate is open. So why didn’t this demogorgon die when the gate under the mall closed?

Every time I’ve looked around for answers online people have said that the russian demogorgon wasn’t connected to the hive mind. But we see in the final episode that when hopper kills it, it temporarily weakens vecna allowing the others to escape from the vines restraining them, so this obviously isn’t the case. So how the hell is it alive?

Someone please explain this to me because i feel like i’m losing my mind.