You can try and play with the font smoothing. Maybe disabling sub-pixel smoothing doves this particular issue. It would produce others, though. Or maybe it’s enough to increase the margin.
I vaguely remember having a similar issue with a different terminal emulator. As I recall the keyword to search for is alignment. I think what’s happening is that the symbol border falls inside a pixel instead of on the border between pixels. This results in font smoothing taking effect, which produces that off-color line.
