Currently, it does not seem like PlaceholderText is not affected by AutoLocalize.
Picture is a comparison of two text objects with the source text “Play” translated to German. Top object is a TextBox with PlaceholderText set to “Play”. Bottom is a TextLabel with the Text property set to “Play”
Yes, that is the case, and that makes sense for user input (the Text property) but does not make sense for the placeholder text that appears before typing (the PlaceholderText property). Developers would want to show a localized placeholder string to players, not always the source one.