""an ending that is portrayed as, and intended to be, a happy ending, but is viewed as a downer ending by the audience"
You list Dollhouse and AI:Artificial Intelligence as examples of the "misused" trope here, but I have seen both and could see how they would fit that quoted description.
There WILL be spoilers in the next two paragraphs, just to warn you:
DOLLHOUSE: This one is, okay, I could see how you'd feel there would be room for debate, but still - it's a "possible example" at minimum, I would argue. Firstly, the world is saved - everyone is Reset Button'd back to their original persona and memories (a good chunk of the theme of the show being that it is wrong to try and "change" people, a theme arguably repeated from Whedon's Serenity film), save for the main characters who deliberately stayed underground for a year or so to stay as their modified selves (also clearly meant to be a victory, since they have undergone lots of growth). The series ends with Priya and Anthony back together and happily raising their child (a real rarity in the Whedonverse!); the characters who have survived before that episode all alive... save for Paul. Whose brain is nonetheless backed up on disc - and installed, by her choice, into Echo. The last scene "they" have together is inside her mind, where they remark on the fact that she's finally "let him in" and opened up to him, as it were, by literally putting him in her head with her, finally growing enough to not be afraid of revealing herself. Emotionally, we're invested in these characters, and it's probably the best ending they could possibly hope for at that point. But. What if Paul and Echo turn out to not be as compatible when inside the same brain? Not to mention, the Fridge Horror inherent in Alpha having gone back to his original, unimprinted persona... people who've read the comics (which weren't all written by Whedon, though they were approved after series' end probably by him) know that the only thing keeping Alpha's psychotic impulses in check was having Paul's mind in there to act as a conscience, so what the hell happens when Paul's mind is gone from Alpha's? We have no way of knowing if the emotional changes will still be rooted there - he could just be the same psycho that was in prison for slicing people up, except, you know, not in prison anymore. Plus, society's infrastructures have been pretty much destroyed and nobody's ever going to trust technology again after the way it was abused. So... what? We enter another dark ages? Or what? It's "happy" in the sense that characters we like are alive and even still together, but what of the rest of the world? Which part are we supposed to consider when considering whether it is a "happy" ending or not - the characters who've been developed, or the faceless masses of the post-apocalyptic world? It's a happy ending by Whedon standards, but there's subtextual or Fridge Logic issues that make it still kinda creepy or... less than happy.
As for AI: I don't even know how to explain how blatantly obvious it was that Spielberg (the guy who finished what was originally a Kubrick film) meant it as a happy ending - the music, the soft focus, it all suggests it. But the explanation in the story is that the resurrected mommy is only going to be there for a day... it's a bittersweet ending, but it's filmed and edited to suggest it's less "bitter" than "sweet". In fact, it was the first example that jumped out at me when the trope was explained here - "oh, like AI". Seriously.
I dunno, it is slightly subjective... but isn't that also fine so long as the reasoning is explained?
Perhaps the description just needs to read, "an ending that is portrayed as a happy ending, but is viewed as [or "viewed as having elements elements of"] a downer ending by at least some of the audience"
None of that "intended to be" business because we're not mind-readers, insert "at least some of" to remove the "but I don't think it was a downer!" etc. silliness.
I dunno, it just... it really seems like a separate trope from just bittersweet endings, because you're talking about something that is FRAMED as a happy ending, but has elements that are less than happy if you actually think about it, yeah? To me, a "bittersweet ending" is openly such; it is framed as BOTH a bitter and a happy ending, as something that has elements of both, front and center. This trope sounds very different from that - something where the surface level or the framing of such, make it "happy", even though there are "downer" elements that aren't being focused on.
This trope also sounds sort of like Fridge Horror - but it's not always horror, and it's always an ending; and sort of like Fridge Logic, but always for an ending and for a specific effect on the interpretation of the ending (making it less "happy").
...possibly a rename could fix a lot of this? "Fridge Downer", perhaps?