I am surprised that you have this problem on Windows. I printed ARROWGLYPH.values() in IDLE Shell and see all solid arrows, [‘
’, ‘⬈’, ‘⮕’, ‘⬊’, ‘
’, ‘⬋’, ‘
’, ‘⬉’, ‘⬈’, ‘⬈’, ‘⬊’, ‘⬊’, ‘⬋’, ‘⬋’, ‘⬉’, ‘⬉’, ‘’, ‘’, ‘⮕’, ‘’], like I do here in FireFox. I am using Source Code Pro. I open the IDLE Settings dialog and selected and applied several different fonts (25?). Nearly all had similar unboxed arrows, with variations in the stem width, sometimes within a font. Some bold or semi-bold fonts did not display a few of them, replacing them with the generic box with question mark. Those few looked to be the ones you had a problem with. Then … Segoe UI Emoji showed the same codepoints with thin arrows in boxes as in your images. That is my only emoji font and the only one with boxed arrows.