It's clearly not fantasy, there is no debate. Fantastical things happen in all three genres, the explanation given for the fantastical things defines the genre. If science is used as an explanation, then it's sci-fi. If magic is explanation then it's fantasy. If the occult is the explanation and/or if there is a high body count and there is an emphasis on scary/bloody events then it's horror. Strangers things uses stuff like alternate dimensions and esp to explain away it's monsters, so it's sci-fi. There are plenty of deaths though and I suppose to children it might be scary, so you might argue that it is horror, but definitely not fantasy as no magic is used and there isn't an orc, elf or dragon in the whole thing.
Orc, elf, dragon, etc. is purely a Tolkien thing, not a Fantasy thing per se.
They can't explain the girl's ability to use telekinesis with science, nor her ability to see into the other dimensions, so this straddles the line.
Is Thor (or any Marvel movie) Fantasy or Sci-Fi? In the movie they state that Magic is just Science that people don't understand. The line is pretty blurry.
Of course Orcs, elves and Dragons are more of a Tolkien thing, but not exclusively. I was joking about that bit anyway. As for Stranger Things....Nope, wrong. It doesn't matter that ESP isn't real in our world, rather that it is real in the fiction and is explained away with science rather than magic. It's Science fiction after all, not science fact.
Thor is neither sci-fi or fantasy, it's a super-hero movie.... different troupes and thematic rules, ect...
Understand that what you, as the viewer, consider the character/film in terms of genre is completely and utterly irrelevant. Ditto in terms of how it is marketed. It's how the author, and as a consequence, the characters in the story, consider the story's events to be explained as that will introduce various plot structures and troupes into the story. In other words the way the story is told is effected and it's the story format that defines the genre.
It's important to make these distinctions because as you point out, Sci-fi has become a dumping ground for any genre stories that don't easily fit into other categories. We can do better than that.
But these are just tools used for critics and scholars to analyze stories so for us dumb schlubs it isn't relevant to the conversation anyway. I only brought it up because Stranger Things is clearly not a fantasy show. It doesn't follow the fantasy show format. There isn't a chosen one, or a prophecy. They don't go on a quest, ect. There's the self-sacrifice of the main heroine which is also present in fantasy shows, but that cliché is found in a lot of genres.