Perhaps you may be able to rebuild the Spotlight Index to see if that re-organizes the index.
• How to rebuild the Spotlight index on your Mac - Apple Support
https://support.apple.com/en-us/HT201716
A secondary (perhaps primary in this instance) indication and resolution may be here:
• Mail for Mac: Rebuild mailboxes
There are two articles associated with the immediate link above, one Mojave 10.14 and other
High Sierra 10.13.x ~ this is the older of those two; your signature line indicates macOS 10.13:
• Rebuild mailboxes in Mail on Mac (10.13.x) - Apple Support
https://support.apple.com/guide/mail/mlhlp1227/11.0/mac/10.13
So there are potentially different aspects of the symptoms involved that could affect your Mac.
