Note that newer Mendix 8 releases, also 8.17, have a fav icon in the navigation profile. The Responsive profile has an icon, new projects have a Mendix icon as default.
Found the original release note:
As long as you’ve replaced the favicon already in the theme folder it should be OK. Favicons are very agressively cached by web browsers, so clear out your caches and do a hard reload, your changes should show.
Hope this helps.