With 46c7a4294babcfed9f0db064e407416b8bfb5b47 applied, KWalletManager shows that the entries it contained under ksshaskpass/passwords/ are now all empty. Prior to this change, the entries under that folder all had passwords stored. With this change, the entries remain present, but they are empty. Checking the "Remember password" checkbox in ksshaskpass's dialog creates an empty entry under ksshaskpass/passwords/. I suspect both bugs originate from the same code path.
Setting to critical severity because user data has been irretrievably lost.
And today not only does saving passwords work, but my old passwords that were missing before are back again. I'm at a loss to explain what happened, but this doesn't seem actionable anymore, so I'm closing the bug report for now.