Missing email content

I use Mac Mail on an iMac - my emails arrive into my inbox fine but when I move *some* emails with attachments to folders I've created in the On My Mac section within Mail, the content disappears. I can see the email in the folder but there's no longer the text content or attachment. I can resend the email to myself from my phone's Mail and it appears whole again on my iMac - until I move it to a folder and the content disappears again. Other emails transfer fine to folders. Possibly / probably related - at the bottom of Mail, there's a constant 'Moving Messages 1 of 653' message. It just sits there. Forever. I spoke to the Helpdesk and upgraded to Monterey 12.4 as they suggested and it's still happening. This has happened to me before - on an older Mac, around 8 years ago. I used another Mac in between and it didn't happen. Makes me feel like I'm doing something wrong but the Helpdesk doesn't think so. If this has happened to me twice, I can't be the only one?