I strongly disagree with some explicit assumptions in the question:
In some cases, I’ve been flagging these questions for ‘proofreading’ when one is wrong grammatically and the other one is correct; this could easily be checked using a website.
Phooey! You cannot look things up on a website unless you know the name of the issue that's confusing you. And if you knew the name of the particular issue, it probably wouldn't be confusing you. It's not easy to look up grammar information on the web unless you already know all the meta-language. And even of you know the meta-language in your own language, it won't be the same in English. And even if you've learned some English meta-language, there's a good chance it won't be the same meta-language as used by other books, websites or scholars.
Proof-reading questions are meant to be off-topic just in those cases where the Original Poster does not highlight which issue or part of the sentence is confusing them. If the OP does in fact give two sentences with just one or two words that are different: that is highlighting exactly the issue that is confusing them. Just because they do not know the name of the issue or how to articulate it conversationally is irrelevant. They have highlighted the specific issue to the readers.
No need to shoot users for assuming one might be correct either, even if both of them are (correct or incorrect).
There is nothing wrong per se with short, simple questions if the issue at hand is clear for readers.