How to make a sidebar form show at the top of page on mobile?

Without knowing more, I can’t tell you if this is a theme setting or plugin update that changed this, or if the code literally was changed, or why the homepage works but the rest doesn’t.

I can tell you from the provided screenshot that the necessary bootstrap that would swap the positions isn’t there. See their reordering guide for bootstrap 4. If you have the ability to edit the template, you should be able to apply this and have it ordered properly.