i see.
i think the issue is the contact form plugin cant handle the same contact form being in the page twice.
because you have a custom phone layout, you basically have the code for the contact form on your page two times. one time in the phone layout code, one time in the desktop layout code.
thats my guess.
maybe you can create a new contact form and insert that one into the phone layout?
and remove the one you already have in the custom phone layout, but without removing it from the desktop layout.
you could also try and disable the custom phone layout feature in lay options.
just to test for a few minutes if my assumption is right.
if you'd disable the custom phone layout feature, you'd not have the code for the contact form twice, so probably it would work on mobile then too.