macOS Mail and sender

Rather than continue a thread from 2020, I’m starting fresh.

While Mail (Version 14.0 (3654. on macOS Big Sur 11.7.10) was displaying my iCloud inbox, I clicked on the New Message icon in the toolbar. A new message window dutifully opened, but the From line showed my edu address. Clicking on the dropdown menu showed all my email addresses except my iCloud address. What’s going on?

I checked Preferences and I could see nothing wrong. (As an aside, both the edu account, which uses Exchange, and the iCloud account have no user-configurable server settings. Didn’t they use to have fields for port selection and other items?) After poking around a bit more, I returned to the draft message. The From dropdown menu now showed my iCloud address. What’s going on?

I sent the message, and it was sent from my edu address (as evidenced by its being in that account’s Sent mailbox), even though I had selected the iCloud address. What’s going on?