I have this problem too. Really wished I had not downladed Big SUr. Awful experience since.

I have a mess now post big sur upgrade. Seacrh comes up with ghost emails that show the incorrect detail in teh previiew pane The titles, email sender and preview content are mismatched. I know the email and all three shown are from different actual maisl. I have tried all of the fixes advised. Reboot, rebuild, etc. Multiple times. If i can not rely on my emails being stored correctly I will eventually have to look for a different sw and service provider for mail! Please Please fix it.