Some of the moderators of this site like to use the "proofreading" close reason when they really mean something like either "I don't think this has enough research" or "I don't think it's clear enough what you are interested in (syntax, natural-sounding usage, etc)". See Andrew Leach♦'s answer to my related Meta question here: Does the “proofreading” close reason cover questions that do identify specific words or phrases?
I'm not a fan of this, because I agree with you that it isn't obviously indicated by the special text for the "proofreading" close reason: "Proofreading questions are off-topic unless a specific source of concern in the text is clearly identified." I would prefer using the "unclear" close option (if it's actually unclear considering the context), or the "show research" close option. But anyway, one person like me cannot control the close culture of this site. The moderators are leaning harder on the "clearly identified" part than I would like (or than you realized).
In practice, this requirement seems to be interpreted by the moderation team to mean that you must explicitly state what subject (like pronoun use, verb tense etc.) your question is about in the body of the question; don't just leave it implicit from the title, tags and examples.
I wouldn't worry about it too much more with regard to this particular question, since it seems to be
on track to being reopened, but in the future, I would advise that you avoid using phrasing like "What is the correct version of this sentence" in your questions, since some people will likely interpret this as meaning your question is a request for "proofreading", even if you think you have provided enough context to indicate that your question is really about syntax, word usage etc. Try to use more specific wording like "Should I use an infinitive or -ing form in this grammatical context" (Yes, I realize that you have had "Verb do + verb to be + ing form" in the title since the first revision, but as I said, even if you think you have provided sufficient context elsewhere just avoid using "correct version of this sentence" wording. It's like the proverbial red flag in front of a bull).