Yes, it’s bad. It’s not super bad, or anything (this isn’t GoT) but the more you think about it, the less sense it makes. It mostly just rushed, not well thought out conclusions. Some conclusions create glaring plot holes (the upside-down explanation makes it impossible for the Russians to have opened a portal to the upside-down, for instance).

Mostly it commits the most egregious sin of not taking itself seriously. End of S4, shit is completely fucked in Hawkins with the ground being ripped up by the massive portals. Beginning of S5, the portals have been tacked over with sheet metal and never once matter.