This seems to have been a “real” space character ( ) added and not CSS padding/margin to the right of the icon element. As a result, it can break some CSS styling that underlines links on hover (e.g., for WCAG compliance).
This (although rather minor) is just as much as a bug as the topic which preceded it:
CSS, specifically the margin property, is the W3C specification on how to handle spacing, rather than by inserting lots of extra spaces within a tag. (Unless living in the 1990s.)
This styling error error effects all Discourse users, even if not visible.
Also, using the CSS styling feature is a normal & typical use of Discourse, in case you were implying somehow this is not a valid bug simply because I switched on the lights to make it easier to see the error.