This bug has started occurring a few months ago, not sure in which plasma-nm version. I'm currently on Debian version 4:5.10.5-2 and it is still present.

When I create an NM entry for a mobile broadband modem and set a per-user PIN code and the PPP password, I expect these secrets to be stored in the KDE keychain.
This seems to be the case, but when making a connection, it looks like the NM daemon fails to fetch them back from the keychain. No error message is displayed, but I get several of password dialogues (PIN, superuser, PPP twice).
After entering all these passwords, the connection succeeds, but this is a very bad user experience.

When I open the NM entry in the connection editor KCM, a popup saying "Failed to get secrets for Connection: No agents were available for this request" is displayed.