Attached is a small project to reproduce the issue. Hovering the mouse over the program window will write messages to standard output describing the emitted events.

In our production code, we have a mouse handler which causes an OpenGL scene to be redrawn when the user presses or releases the alt key with the mouse pointer hovered over the scene viewport. Similarly, in the reproduction project, I have put a call to QQuickItem::update in the custom key press and release event handlers. When pressing and releasing alt, I get the following output from the test program:

Pressed key alt!
Hover move event! With no modifiers
Released key alt!
Hover move event! With modifiers: alt

So an "outdated" hover event is emitted after the alt key has been released. The production component in question has recently been migrated from QtWidgets to QtQuick, and we have not experienced this issue with QtWidgets, so there seems to be a divergence in behavior.