Apple Mail messages with no content

I am receiving email messages with headers but no body (content). When I try to forward the message I get content from an adjacent message. I booted into Safe mode and these symptoms go away.

I have been updating the OS via Migration Assistant for decades now and my work required a lot of experimentation with new software so my system is probably full of cruft.

Since I cannot stay in Safe Mode forever, I need to ID and remove whatever non-Apple code is causing these symptoms unless there is a system-wide cruft removal process that I can use.

What to do?