For many users Mojave email will periodically open in Split View mode even then Mail Preferences: General split view is unchecked.
I'm trying this fix:
- Check Mail Preferences:General "Prefer opening messages in split view when in full screen". Confirm Mail opens in split view. Maybe quit and restart.
- Now open mail preferences and uncheck that option and force quit Mail. (Somewhere in the Mojave era or earlier macOS preferences got wonky such that an app on exiting could do weird and occult things to preferences.)
- Restart mail with Safari open in full screen and confirm you don't get the Split View -- get Mail as full screen.
Works so far but I wouldn't be surprised if fix doesn't last.