The problem is that one/two¹ of the example sentences for 見返す don’t have a direct object marked with を, while two of the example sentences for 見返る do have an object marked with を. I think it’s especially confusing that for 見返す it’s about a kitten looking back at me, and for 見返る it’s about a baby looking back at me, which makes them seem interchangeable².
When I googled “difference 見返る 見返す” I found an answer from @TofuguKanae and two answers on HiNative:
I think it’s interesting that neither of these native speakers mentioned transitivity.
And here are two other WK forum threads with the same question:
¹ In the sentence “答案は提出する前に見返したほうがいい。”, 答案 is the direct object, but it is marked with は instead of を. So this sentence does have a direct object, it is just omitted to avoid repetition (答案は答案をみかえす=> 答案はみかえす). At least that’s how I interpret it.
² Maybe the difference is that in the case of the kitten, it first looked at me, then at the sardines, and after that it looks back at me again, while the baby just looks at me? Probably not, and I’m just trying to construct a difference where there is none.