One standard user-error is that users search for terms but have the wrong filtering active. I type a file-name, but the active filter is "LABEL CONTAINS". Obviously, this Search retrieves no results (and Developers can't do anything about this).

Now I get aware of my error and edit the Search filter. I still want to search for a file-name which is already input, so all I need to do is switch to the proper file attribute via Dropdown. I pick Document-Name and run the Search again.

This new Search will again spit out no results (and this time Developers could do something about this). Whenever users switch to another Search Criteria (from Labels to Document Names) existing textual input gets deleted → the perfectly fine file-name is gone. This is deplorable UX – it's so bad that I even run into this trap regularly – although I know it's there.

When filtering for Keywords it's error-prone and inconvenient to let users type the Keyword. Instead you should offer an auto-complete and display the filtered KW as nodes (see image), in the way your Custom Metadata Panel does this.