Well, that works but it’s still just a workaround which… doesn’t feel right.
A dynamic zone should be used for fields for which I can fill out different kind of components.
(I use this for blog posts - user can select RichText, or Quote components for example).
But in this case, where I have optional primaryButton field, turning it into dynamic zone in which I can only select on kind of component… again… feels weird.
I don’t know if I am missing something, or it’s a Strapi bug and I should open an issue, or it’s intended behaviour (if so, it’s really weird behaviour in my opinion) 
Simply, if the field is optional, I don’t see a reason why should it’s fields POP UP after saving an item.