I set up quickly a testing tenant with the blog recipe. Then I wanted to create quickly a blog post from the admin UI. I typed a title only and pressed the enter key and it opened up the "Markdown help" page in a new tab. When we press enter it should submit the current page form.

Hi! Could you please give more details to reproduce this bug?
Try to create a content item that has a MarkdownBody Part. So basically the Content Type needs to have a MarkdownBody part attached to it.
Sorry, I'm new to this project, if you could explain me how to get at this screen where is the problem, please. Just a few steps to help me to get at the point where the problem happens.
If you run the blog recipe to start your site setup, you will see the problem when editing the first blog post.
Note: We need to check the markdown field as well (but later @eddamazioribeiro )