My controversial publishing take is that proofreading isn’t actually an entry-level position, and publishers should stop making it the “starting point” for getting into publishing jobs.
Proofreading needs a firm grasp of language, strong attention to detail, and knowledge of industry standards and changing trends. Most people—even authors and editors—don’t know the ins and outs of grammar rules and style guides (nor do they need to! That’s why this is a job!)
Novice editors, especially when proofreading or copyediting, often over-edit. I’ve heard many authors say, “They completely missed the point” or “They changed the entire thing.” This is part of what makes training important.
You have to know the publisher’s guidelines, industry standards, and common vernacular, but you also need to honor the author’s voice, vision, and style. These may be good consideration questions, whether you’re editing, giving feedback, or even reviewing someone else’s suggestions for your own writing.