Website passwords in keychains deleted after Catalina update

After upgrading to Catalina, the website passwords -- but not app passwords, secure notes, certs, or keys -- were deleted from all keychains in `~/Library/Keychains`. Keychains stored elsewhere were not affected.

I use different keychain files for different purposes (personal, work, etc.). Many but not all of these are stored in `~/Library/Keychains`. I can see in the filesystem modification time that all the keychains in there were updated as of the time I upgraded to Catalina although many of these keychains haven't been unlocked in months. It seems there must have been some process as part of the upgrade that deleted things.

I ran the unix utility `strings` on the keychains which usually can reveal the names of items in a keychain database file. Unfortunately, it looks like the passwords in question have been completely deleted.

Notes:

• MacOS Catalina 10.15
• MacBook Pro (Retina, Mid 2012)
• Safari Version 13.0.2
• iCloud *not* used to sync the keychain

Possibly related: https://discussions.apple.com/thread/250700942