I had a look what Scintilla does with copy/pasting rectangular selections and probably have an explanation for what happens. Scintilla has to distinguish somehow whether the text in the clipboard is a rectangular selection or ordinary lines ending with `\n`. It does this by placing `\0` behind the `\n` for rectangular selections. So for example ``` abc\n\0 abc\n\0 abc\n\0 ``` in clipboard is a rectangular selection, ``` abc\n abc\n abc\n ``` is ordinary selection.
I assume that macOS just doesn't like the `\0` characters and drops them which effectively changes the rectangular selection in clipboard into ordinary selection.
Maybe on macOS some other character could be used, e.g. `\1`, I'm just not sure how hard it would be to make this change in Scintilla.