QWidget::childAt always returning null

After creating a matrix of widgets with QGridLayout, using QWidget::childAt inside mouseMoveEvent returns null most of the times.

The weird thing is that it returns valid pointers only after calling event->screenPos().

What's even weirder is that enabling mouse tracking on the widgets in the grid fixes the problem.

I have attached an example project reproducing the bug.
You can get the expected result un-commenting line 22 in MainWidget.cpp (enabling mouse tracking for the buttons).