Disappearing form type/template

I’m a bit struggling with a contact form.
I’m using dutch nl as my language setting.

I’m only using the default forms and email plugins. So nothing fancy/strange with that.

The contact form itself works flawlessly.
But to get it working i needed to create it with the antimatter theme because i couldn’t select ‘Form’ as page type in the admin.

After creating the form, i could change to my own theme, and it works.
But if i edit the page, the page type changes to another type (in my case blog) and the form is not selectabe anymore.
It’s disappeared?!

There’s no page template/file for a form, so it should be default
I followed https://learn.getgrav.org/forms/forms
(i’m trying to create my own theme based on Haywire. )