DriveItem search not working properly

Question

I used the following search API and got an empty result, but in fact there are files that match the search criteria.

/me/drive/root/search(q='{search-text}')

1. /me/drive/root/search(q='Doc') -> I got files
2. /me/drive/root/search(q='Docu') -> I got empty result
3. /me/drive/root/search(q='Docum') -> I got empty result
4. /me/drive/root/search(q='Document') -> I got files

On my personal drive I have files like this, for example: Document.pdf, Document 1.pdf, ...

Could you please tell me what the problem is or what I am doing wrong?