I have a similar problem with keychain. On iMac (21.5-inch, Late 2013). I have upgraded the OS to Catalina. The upgrade went well, no error message/alerts.
After a couple of days, I opened keychain to retrieve an account login & password. It was missing. I noticed others. I HAD many internet accounts, manually entered in keychain. Out of 388 items in keychain -- 108 items were gone. I used Time Machine to restore my keychain from 1 day prior to the installation of Catalina and added it to my keychain as a secondary (not default) keychain. All my keychain items were there (much to my relief). However, after logging out of the iMac and back and immediately opening keychain, I literally witnessed the same items disappearing as the remaining items scrolled up to replace the records as they were being deleted.
Because I have not yet found useful information regarding this issue, I began eliminating possibilities. I unchecked "Keychain" from Apple ID (formerly iCloud) in System Preferences, replaced the semi-destroyed keychain with a copy, logged out and back in -- same result 108 items disappear.
Only once during several similar efforts, did I get a "Reminder" message similar to the one "moi160" described in which the secondary (good) Keychain was referenced and which I captured the following screenshot:
This has only appear once during my trials. Coincidence?
There appears to be a greater problem with Keychain for those "rare"? users who actually use Keychain to manage their internet urls, account names, and passwords. MY records seem to have taken a backseat to (background) system calls attempting to manage my internet security (which I appreciate). However, PAY ATTENTION APPLE -- this is going to be a problem as users realize their essential user data is being wiped out of Keychain.
I am especially grateful for the Time Machine backup of my keychain. TM is CURRENTLY my salvation. Under no circumstance, should a user's data be deleted from Keychain without user confirmation. At minimum, a choice to export the user's data to a deprecated keychain should be implemented.
Sorry, I'm not knowledgeable enough to know how to resolve this issue. I also failed to identify this problem as a Beta tester of Catalina. Now I can't go back, Time Machine has been Catalina-ized to the dual "Macintosh HD" plus "Macintosh HD - Data" volumes. (Apple developers: you know what I'm talking about.)