Favorite servers aren't being saved to file
I guess this could cause bug to happen https://github.com/FWGS/mainui_cpp/blob/464060552121f5b51c863b517b31ae5b4770c30a/menus/ServerBrowser.cpp#L982-L986
Are you adding server by IP without pressing "Favorite" after?
Yes
That's not how it works and TODO comment explains that.
Servers entered by an IP address (let's call them "custom" servers) get added to Favorites tab, because there is no suitable logic to do this with Direct games tab.
And because custom server might not be your favorite one, to make it so, user have to press Favorite button. All favorite servers are marked with an asterisk icon.
I tried it with GoldSrc server, and it does not being saved to file even without marking as favorite.
P.S: Retried it, and for this time it worked, lol.
You specifically need to mark it as favorite and then exit menu through Done button. Or connect to some server, it should work as well.