The square box indicates that one of the paragraph pagination settings is turned on (either keep with next, keep lines together or page break before). It is not relevant to your search and replace failing.

You might have tracked revisions sitting in there though which breaks up the three consecutive paragraph marks. Can you verify if there is any tracked revisions in the document?

Also, the content could be protected in some way - locked Content Controls, grouped text or document forms protection for instance.

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Andrew Lockton
Chrysalis Design, Melbourne Australia