This looks like a regression in 5.15.2.

Scene3D can be configured to OnDemand rendering and it works fine for 3D entities,
but any other items are rendered at every frame.

STR:
1. create Scene3D and set RenderSettings.OnDemand (see attached empty scene)
2. run qmlscene ondemand-scene3d-updates.qml by RenderDoc
3. Check fps and/or capture frame to see which items are rendered

First bad revision is
09eb4028e "Rework Scene3D to fix potential crash on shutdown"