Sluggish mouse wheel scrolling in Flickable and ComboBox drop down

Scrolling using the mouse wheel in a ComboBox drop down feels very sluggish on my Mac. Sometimes it doesn't react to the wheel being spinned, sometimes it scrolls very slowly, sometimes it jumps around.
Scrolling using dragging (click + drag) works smoothly.

Sometimes I also manage to scroll the items completely out of the view and then Qt start emitting these errors:

file:///usr/local/Qt/5.15.0/clang_64/qml/QtQuick/Controls.2/ComboBox.qml:56: TypeError: Cannot read property 'width' of null

This typically happens when I start scrolling quickly after clicking the ComboBox for the first time. But it doesn't happen every time.

Attached you'll find a minimal example to reproduce the issue.
It's simply a ComboBox inside an ApplicationWindow.
Run it with

./qmlscene /path/to/comboBoxScrollBroken.qml

I already played around with the --scaling/--no-scaling, --gles, --software and --disable-context-sharing parameters of qmlscene but it doesn't seem to change anything.
Setting wheelEnabled: true on the ComboBox also doesn't seem to have an effect on the scroll behavior.

Flickable (like ListView or TableView) basically shows the same behavior. However, there it works a bit better. Meaning: as long as you scroll slowly and don't scroll to the end of the view, it works okayish.