paste '☆' into your mintty prompt. It will appear as only left half of the character. Some other full width characters render correctly, like 你好.

The star character displays a correct full width character when I am using urxvt on linux. This happens for a handful of other characters as well but I don't have a list of all of them off hand. tested with 2.5.0 from msys2 and 2.6.2