This isn't a rant. More like a whine. I'm beta reading a novel for someone I met online, and in the process I'm coming to doubt my own novel, my ability to distinguish between good and bad writing, and my perceptions of life in general. Without giving so much detail I breach confidentiality with the author, let's say I'm having trouble figuring out the main character. I'm a dozen chapters in, and I still don't know who she (basically) is. Is she bold and gregarious, or studious and reclusive? A cool, distant observer, who seems to regard people as lab rats and preparations, or a tender-hearted humanitarian who'll leap in to save a human stray puppy? A repressed, naive child who seems to think she can get married without ever forming a friendship with a young man, or an unselfcensored woman who'll readily bring up sexually-tinged matters about herself with guys she barely knows? Or all of these at once? Very often, it is all of these at once. The MC changes from one personality to another in the course of a single paragraph, and I keep wanting to ask the author, "Why is she reacting like that? Didn't she just say she was afraid of/didn't care for/never did and wouldn't do [whatever]? And now she's saying it's fine with her. And look, she doesn't even notice the contradiction!" But then I step back. I think, "Gosh, who am I to say her character can't act like that? Just because I wouldn't do/react like [whatever it is] doesn't mean this character can't." Gosh, maybe I've got bizarre, inconsistent stuff like that going on in my own book and don't realize it. If I'm that clueless and blind, perhaps I can't even trust myself to make valid judgements about the world around me! Aaaagggghhhh!!! This beta-read has my head all mixed up. It's depressing and beyond. [End of whine.]
I believe I understand where you're coming from. Perhaps it would help you if you just distanced yourself from your own reaction and perceive it objectively. If you do really stumble while reading those contradictions, then it is quite possible other readers will as well - therefore it is definitely a valid point of criticism. A contradiction within such a short amount of time is certainly not a good thing - yes, people are complicated, but no. If she did state she definitely wouldn't do X, and then suddenly does it a few lines later, it IS a contradiction. Unless, of course, being a hypocrite is part of her character, which I suppose you'll find out later if it is indeed the case. In any case, I wouldn't worry too much about it if I were you. As a Beta Reader, you are supposed to give the writer your genuine reactions. If you think it doesn't work, then it probably doesn't.
Like @ReproveTheCurlew said, your job as a beta reader is to give your impressions and thoughts. It's helpful if you explain your comments ("this strikes me as inconsistent because in the last chapter she said she was scared of authority, and now she's talking back at her teacher like she's been doing it all her life") but it's not your job to analyse WHY you have those thoughts or to suggest fixes. It's up to the author to take your feedback, determine if they've achieved the effect they wanted and, if they haven't, to dig deeper and find out why. My suggestion? When you're beta reading, forget you're a writer. Pretend you've just bought this book and are reading it for fun.
I agree with the Bird Scolder - a beta read is supposed to be about your reactions. You're not an editor, you're a beta. If something doesn't feel right to you, that's what you should put down. In terms of questioning your own writing? Good! Question it, and get the answers. Ask your characters - are you being consistent? And if they aren't, do something about it!
Sage advice. There is only one teacher, you yourself. To quote someone I never thought I would find myself quoting, no one can learn it for you.
I think you should, ask those questions that is. See good writing shouldn't make you pause. It shouldn't make you challenge who they are. You are beta reading. The point as far as I can see is to help them see the difference between what they have and what they want. So it isn't rude. Perhaps the author didn't mean for them to be so different each paragraph. In which case you have helped them learn and grow. Perhaps though, they did mean for that to happen. In which case you are telling them they got what they wanted. Which is also nice.