If messages won;t transfer, or Mail shows other inconsistent behaviour, try rebuilding Apple Mail’s database by deleting envelope index files...
Quit from Mail, and return to Finder.
Open the Library folder (either make it visible, or hold down option key while using the "Go" menu, or use the "Go to Folder" command).
Open the "Mail" folder. Open the latest "V" folder (it is "V9" for Monterey, "V10" for Sonoma)
Open the "Mail Data" folder.
Delete all files that begin with "Envelope Index"
Optionally remove saved application state as well. Go to "Saved Application State" in "Library" folder. Find "com.apple.mail.savedState". If it's an alias, find the original and delete it, otherwise just delete it.
Launch Mail again. it will show a window "Message Import". Wait for this to complete.
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