Steps to reproduce
when doing a return at the end on the main title, it's not adding a line between the title and other paragraph under.
introducing a new line right after the title or sending the cursor to the beginning of the next line
The ‘title’ at the top of the new V1 editor is the file name, not an H1 title. I think that’s what is showing in your screenshot.
Obsidian > Preferences > Appearance > Show inline title