It's possible the Collected Addresses address book, or whichever book is the collector, is corrupted, as such books typically do not accept new contacts or changes to existing ones. If you manually add a contact to CA, then restart TB, is the contact still present in CA?

That solution seems to have worked. Strange thing is that I have been using Thunderbird for years and there should be dozens of addresses in the "Collected Addresses". But when I looked there were only four. Hopefully this "New Contact/Restart" solution continues to work.

If Collected Addresses, or any other address book, is corrupted, the usual remedy is to export the existing contacts to an LDIF file, then open the profile folder (from Help/Troubleshooting Information, Open Folder), close TB, delete history.mab (Collected Addresses) or abook.mab (Personal Address Book). Upon restart, TB creates a new, uncorrupted book, and the LDIF file can be imported and the contacts moved by drag-and-drop to the new book.