Keychain Access won't show login keychain passwords

I need to retrieve a password for a Pages document on my M1 iMac, so I opened Keychain Access, selected the 'login' keychain, found the document in the 'Passwords' list, double-clicked it to open its info page, and clicked the 'Show password' checkbox. As expected, the "Keychain Access wants to use the login keychain. Please enter the keychain password." dialog popped up, and I entered the keychain password. I believe the password is my local system login, since I haven't changed it.

Keychain Access accepted the password--there was none of the jittering that occurs when an incorrect password is entered, and the dialog closed. However, nothing else happened--the password I need did not appear in the info page. I repeated the process a couple of times, with the same result.

Next, I tested my assumptions by repeating the process and entering an intentionally-bad password. As expected, the password dialog jittered and did not close when I clicked the 'Allow' button. So, it appears the password is correct and is being accepted by Keychain Access, but the password display process is failing to complete.

I checked my iCloud Keychain using the same process--Keychain Access accepted the good password, and in turn, it displayed my password for the site whose record I selected. So, the problem appears to be isolated to the 'login' keychain.

As I was performing these diagnostic tests, several dialogs popped up, apparently from system processes that wanted access to the login keychain. For example: "Transparent network proxy for Apple system

services wants to use the "login" keychain. Please enter the keychain password." The dialogs accepted the login keychain password, apparently without issue.

At this point, I'm stumped--here are my questions: Is this a known problem? Any suggestions for fixes or further troubleshooting, or do I need to delete my login keychain and start from scratch? Thanks for your help!