The Duplicate Question search used to search based on text entered into that field.
Now it doesn't even find an exact title match, and as the text suggests it now requires a Question ID:
The behavior has been changed.
My question is why was the behavior changed? The search feature helped us to find duplicates without having to go to another window.
Is this an oversight? If so, this is a feature request to include the inline search as it was before.
Note: Because I voted to close the question that precipitated this post without recording a screenshot, I wasn't able to reproduce it for that question so I picked another question (that isn't a duplicate).
Edit: It looks like the feature to search inline was removed when the new version of the "Exact Duplicate" close feature was implemented. Recommend adding back the 'search by question title' (as well as what's currently there).