Do you remember old style physical books? If you want to find something in a physical book there is no search function but only an index. Instead of re-reading everything from the start you can look up a word in the index and the find the location you are looking for. Databases rely on a similar index function to find data.
If the index is screwed up then you don't find the correct data.
If you can't see the correct content in an email it means that the index of the email client is screwed up. It's then necessary to fix the index.
A while ago I had a look at the structure of the data of Mail. In there is the general method of fixing problems with Mail:
- Quit Mail.
- Go to user/username/Library/Mail/Vx/MailData.
- Move the MailData folder to the desktop.
- Restart Mail.
- Wait until Mail has finished rebuilding the index.
Vx is a folder where x is a letter depending on the version of macOS. This is the folder on Big Sur that needs to be moved to the desktop: