I stumbled onto a workaround after months of frustration. I migrated from a 2017 MacBook pro to the new 14" version and could not search any of my emails sent or received prior to the migration. I tried everything in this thread and nothing worked. I found that if I selected a folder, then "select all", right-click, "follow up", "mark as complete", after a few minutes I could again search all of my emails. Not a tech-savvy solution, but it works.
RoscoeC
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