In Qt5.15.1 geometryChanged signals are not emitted if XCB_RANDR_SCREEN_CHANGE_NOTIFY and XCB_RANDR_NOTIFY arrive together.
The geometry property is still updated
See bug QtBug - 86604
This disproportionately affects plasma as we are quite screen-dependent and are active when screens are doing the initial startup where both of these properties change. So I think a temporary workaround is worth considering.
This hack emits the missing signals from what is emitted. It will result in a bit more noise, but most implementations should just no-op.