Can't read old email messages on new iMac

I have some old emails saved from my last iMac (late 2013) which was running Catalina 10 something. I used to use Find to retrieve information from them. I just bought an M1 iMac (2021) which runs Sequoia and although I can use Find to retrieve these old messages and open them in Mail, all it displays are the headers. The text content (body of the message) does not appear. I just get white space.

Any help?