Created attachment 158586 [details]
test.qml

SUMMARY

tl;dr KQuickStyleItem does not watch for changes to focusReason, in case it is set after focus is already set or lost.

In QtQuick every Item type has focus and activeFocusOnTab properties; but Controls also do have focusReason status and visualFocus derived from it. For keyboard interactions it is common to set TabFocusReason on the receiving control and BacktabFocusReason on the previously active. However, some technical limitations such as https://bugreports.qt.io/browse/QTBUG-97006 might force developers to resort to workarounds like setting ListView's current index (thereby making some other item the currentItem and giving it activeFocus) and then setting focusReason manually (also note that invoking forceActiveFocus() won't change the reason as it short-circuits because an item is already focused):

    contentItem: ListView {
        // Roll out custom key navigation because built-in one sets wrong focusReason.
        keyNavigationEnabled: false

        delegate: QQC2.Button {
            Keys.onDownPressed: {
                ListView.view.incrementCurrentIndex();
                ListView.view.currentItem.focusReason = Qt.TabFocusReason;
            }

            Keys.onUpPressed: {
                ListView.view.decrementCurrentIndex();
                ListView.view.currentItem.focusReason = Qt.TabFocusReason;
            }
        }
    }

It works fine for Plasma controls and for any pure QML custom styling, but not for qqc2-desktop-style bridge item, because this:

bool KQuickStyleItem::eventFilter(QObject *watched, QEvent *event)
{
    if (watched == m_control) {
        if (event->type() == QEvent::FocusIn || event->type() == QEvent::FocusOut) {
            QFocusEvent *fe = static_cast<QFocusEvent *>(event);
            m_lastFocusReason = fe->reason();
        }

this is the only place where `m_lastFocusReason` is updated. Hence, changing it after FocusIn even at runtime won't have any effect on the appearance of a control.

STEPS TO REPRODUCE
0. export QT_QUICK_CONTROLS_STYLE=org.kde.desktop
1. Open this snippet (also available as an attachment) with qml or qmlscene: https://invent.kde.org/frameworks/qqc2-desktop-style/-/snippets/2633
2. Navigate using Tab and Shift+Tab keys.
3. Navigate using Up/Down arrows.

OBSERVED RESULT
Buttons have nice highlighted outline when receiving focus via Tab, but not via Up/Down keys.

EXPECTED RESULT
Focus highlight outline effect should applied regardless whether focusReason was set at the time of FocusIn event or at some point later.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: git/5.27
KDE Frameworks Version: git/kf5
Qt Version: 5.15.9