[Update]
See my comment about your feedback on Discourse’s own forum here.
Hi @p.kayam,
Thank you for your feedback. 
Apparently, the search banner mimics the search button’s behavior in the upper right corner in that:
- Pressing Enter once brings up the dropdown list with the most relevant search results.
- Pressing Enter twice brings up the Advance Search UX in which you can further filter your search results based on date, status, and more. You can refer to this topic to learn more about
using Advance Search.
I believe this is a current limitation with this theme-component by Discourse (a lot of the UI/UX elements you see around the forum were created by them as well). I will let them know about this