The Pages Find/Replace tool does not operate off of any selected words, but rather will scan the entire document for the word that is in the Find field. This defeats focusing the Find/Replace operation on a single block of selected text.
If you previously copy/paste a word into this field, it will remain on the search options clipboard and automatically appear in the Find field, the next time you use it, which would require you to manually update that field with the current text to be searched.
If the Find/Replace panel is set to Match Case, then searching for a lower case word when the body text word is title/upper-case text, or enclosed in quotes, will fail. If Whole Words and not Match Case is selected, then nothing will be found until you click the right-arrow in the Find/Replace tool — regardless if the word is quoted or not.
In practice, the use of the Find/Replace tool in Pages v11.2 is now, as described above, working differently than described in the Pages User Guide for Mac's entry on Find/Replace.
Tested: Pages v11.2 and macOS 11.6 on M1 mini.