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.
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Plasma 6 Bugs
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thanks for the answer! the only settings about alignment I found in konsole are under edit profile > appearance > miscellaneous > align to center, but it does not seem to do anything to the fonts
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.
Try changing your font size.
I installed a few nerd fonts yesterday. The first one looked just a bit weird in Konsole, the second one looked fine