Search within specified folder gives results but within 'This Mac' gives none
By default, Search 'This Mac' excludes System Folders

By starting your search inside ~/Library/ you are forcing it to bypass that default, as you are already inside a normally-excluded location.

To persuade it to add System Folders to a 'This Mac' search, you must specify as an additional parameter.
Click the + sign, top right [which then turns into a - ]
Add 'System files' &amp; 'are included' to your search parameters.