Thanks for the response. The scenario is: I use Blue Host to host my WordPress website. I have a live and a testing (staging) site. Up until a few weeks ago, the Staging site was an exact replica of the live site (using the Twenty Nineteen theme).
I have now decided to upgrade to Twenty Twenty Three which I have done on the Staging Site. All the Posts and Pages on the Staging Site look fine, although a few tweaks are necessary. I have added a WordPress Search block in the Footer template on the Staging Site. Ordinary text in the posts and on the pages return correct results but when I search by post author, such as John Smith, John or Smith there are no results. If I create a new user and a new post then the search is fine when searching for the new author of the post. So, it seems that the problem lies with original posts created in Twenty Nineteen. Has anyone else come across this problem? Is there a simple workaround or is this a bug which needs to be addressed?
Many thanks.