Many thanks for your feedback — happy you like it!
Your probably observing this in the context of renaming files. I've made some subtle changes on how special characters are treated with the Mac version:
Invalid characters are automagically replaced by an underscore on field access, and by that, circumvents what I call the "AC/DC"-Problem. In the past, renaming with fields that contained a path separator char — it's the forward slash on macOS — would result in unwanted folders being greated, e.g., "AC" and "DC".
Invalid characters are currently / and : (and some newline and control characters) and if you have the NTFS option enabled at Preferences > General, the list of special characters extends to the invalid chars on Windows.
So, it's unfortunately not possible to distinguish between the underscore that results from the automatic replacement of the : character and the one that is the result of the / replacement.