When a password backend is configured, regardless of KWallet or integrated, 'auto complete password on sites' will insert the password in non-password fields (like date, search) on https://app.obono.at. Not random fields, but always the same. This was observed only with Falkon (multiple versions and Qt versions); other browsers such as Chromium and Firefox are fine. I didn't notice this behavior on any other site yet. Even without logging into the account, the password will be displayed as login name rather than (and in addition to) the password input field.

Versions tested so far:

Falkon 3.0.1, Qt 5.9.4 (openSUSE Linux x86_64)
Falkon 3.1.0, Qt 5.13.2 (Gentoo Linux x86_64)
Falkon git Falkon/3.1 @ bfb9420c, Qt 5.13.2 (Gentoo Linux x86_64)
Falkon git master @ 2d5bf8d4, Qt 5.13.2 (Gentoo Linux x86_64)