I did not touch anything, that’s really weird because here above also the icon appears in onebox 
Edit: In fact I maybe get an idea, but I don’t know if it’s a start of an explanation: I changed the generic template in order to have favicon absolute URLs in heads, and forced this particular page to update the template, but not other pages (which were to be updated later depending on their cache expiry)… Maybe discourse get the favicon information from the non updated landing (root) page source and not from the pasted URL source itself?