Selecting plain (non-link) text in the target column shows two link cards: source and target.
Showing the source link in such a case seems unnecessary. It causes T102115, which definitely must be fixed, because links cannot be added to the source wiki. But I think even further - the source card probably shouldn't be shown in such a case at all.
There are several cases:
# The page with the selected title doesn't exist in the target wiki. In this case, there should be a possibility to add a red link (although this may become different after T89953 is done), but I don't see a reason to show a source link card.
# The page with the selected title exists in the target wiki. In this case it makes sense to add a link to it, but which source card should be shown, if at all? "Gos", for example, is "Dog" in Catalan, so if I select "Gos" when I translate from English to Catalan, which source link should be shown en.wikipedia.org/wiki/Gos or en.wikipedia.org/wiki/Dog ? I'd argue that neither is really needed, although maybe it's good for design to show two cards whenever possible for consistency whenever possible. @Pginer-WMF, what do you think? And if the source article doesn't exist, is it useful to show anything?