If a contact appears on page 1 of the contacts view, it cannot be merged with a contact on any other page. This is because when you flip the page to the next batch of contacts, the selection is lost. That means that you can only merge contacts that are within range of about 50 contacts of one another. What a silly loss of functionality.
Thanks for the feedback @dedicatedpresentatio!
You’re right that selection doesn’t carry across pages, and that’s worth looking at on its own, though it shouldn’t block you from merging duplicates.
Search is the way around this. In Contacts, search for the name, email, phone, etc. and all relevant contacts will be shown together in the filtered view. You can now select duplicates and merge as expected. Since duplicates share identifying details, this works no matter how far apart they sit in the full list.
Our merge guide doesn’t mention the search step, which is why this looked like a dead end. We’re fixing the article.