(Thanks for clarifying.)
No, titles do not need to be formatted as questions. They just need to communicate well what the question is about. They're titles! If you want to know what the question is, read the question body. That's the place where all the important context is, too, and you should be reading that.
(If you're not, you're answering the title alone, and that is bad practice and a fast way to create an unhelpful answer. Either it will be obvious you didn't read, or you'll just miss all the important details and create an answer vastly inferior to what you could have created.)
But, as much as it tickles me, are these titles bad? Maybe it bothers me because I'm a fan of the Q&A format, but I get triggered because I don't see a question.
They're not great, but not because they're not full questions.
"Reversing AC rolls?" is a confusing title: what's an AC roll, what's this concept of 'reversing' it? Really, this question is about going from a model of attack rolls vs AC to attacks vs an AC roll, to be in line with saving throws, so the new title What are the implications of using AC rolls for defense? is clearer. If I were looking for that kind of thing in search results, I'd be grabbed by the new title, but not the original.
The other two titles are, also, each just a couple of words away from something more thoroughly communicative. That turns that into an easy improvement. "Pack Flanking on non Hunter Character?" - what about it? It's an easy shot to edit that into the exact nature of what's being engaged in: How can I get Pack Flanking on a non-Hunter character?. I also like seeing titles that makes sense on their own and use decent grammar.
The improvements here are not because we're turning those into questions. We could turn them into statements if we wanted to. The improvement will be that we're making them more communicative. If putting a brief question in the title helps communication, so be it, but titles that are statements are not incorrect.
So in summation:
Should the question title reflect the core question of the question's post? Should that title be formatted as a question, or is it okay if it's a statement, or even just a bunch of keywords?
It should reflect the core of the question. It is okay whether it is a question or a statement.
If a title communicates poorly it should be improved. Titles that are just a bunch of keywords usually communicate poorly.
Edit to improve communication of what the question's about.
Don't expect a title to contain the entirety of the question. If you want to know what the question is truly, you should read the question body. The title just needs to communicate what the inquiry is about, and statements do that just fine.