Based on our troubleshooting, it could be a possible bug.
You could try first re-building the application.
To rebuild a single or multiple Apple Mail mailboxes:
- Open the Mail app, and then, select one or more mailboxes.
- From the Mail app's menu bar, select Mailbox > Rebuild
To rebuild the Mail app message database:
- Quit Mail (if running)
- In Finder, navigate to:
- ~/Library/Mail/V3/MailData
- Drag the following three files to the Trash:
- Envelope Index
- Envelope Index-shm
- Envelope index-wal
- Relaunch the Mail app.
If this will not work, you can try re-installing the app by re-installing your macOS. But I think it is better to inform Apple about this by contacting their support.