Custom ScrollBar style not used after upgrade from 5.15 to 6.2

This is a regression introduced when upgrading from Qt 5.15.2 to 6.2.2.

I have my own style for QtQuickControls as described in the docs. I do not need to override all components, so I have a fallback in my qmldir file:

module QtQuick.Controls.MyStyle

Button 6.0 Button.qml
# ...
ScrollBar 6.0 ScrollBar.qml

# Fallback
import QtQuick.Controls.Basic auto

When I use a ScrollView (which I do not override from the Basic style), it does not use my custom-styled ScrollBar but the ScrollBar from the Basic style.

I would expect it to use my custom-styled ScrollBar. This was also the behavior of Qt 5.15.2.

Suggestion for a fix

These are the imports in qtdeclarative/src/quickcontrols2/basic/ScrollView.qml:

import QtQuick
import QtQuick.Controls.impl
import QtQuick.Templates as T

It currently imports the ScrollBar from QtQuick.Controls.impl, so it does not get styled.

If the imports were to be changed to the following, it would pick up the custom ScrollBar style:

import QtQuick
import QtQuick.Controls
import QtQuick.Templates as T

Workaround

In the meantime, this can be solved by adding a ScrollView to the custom theme.