Use fallback resolver for nxdomain

Hello to all,
I wanted to ask you for information and sorry for my ignorance about it.
Why does the option use fallback resolver for nxdomain not work in the app 1.1.1.1 for android, that is, if you click on it, it does not enable and instead for iphone it works fine? why this thing? is there a particular reason?

Why do you keep deleting and reposting?

I deleted by mistake,
can you tell me if it’s a simple android app bug?