A link that isn't to an external page is treated as external link. Functionality is broken (it opens in a new tab) and formatting is wrong (external link icon is added to it).
Links that don't conform to a certain pattern are changed to be formatted as external links (by adding an svg icon and a class name) and target="_blank" gets added the them. The function that does this is greedy and doesn't exclude this case. In this case, the link should not be treated as an external link.
Reported at Github