Plugin setting page – update_option problem

Your chunk of code if (isset($_POST['submit'])) { would process all the form submissions (even other forms which is not your settings page) which is why you are lossing your data. Even though this is not good way to create option page.

But

If you do something like

if(is_admin() && isset($_POST['submit']) && isset($_POST['ffita_gads_capub'])){....

could solve your problem.