Maybe one of them was down. Either physically or in QEMU.Parameters
wParam
Indicates whether various virtual keys are down. This parameter can be one or more of the following values.
I do not know much about virtual keys, and how much of them actually exist/whether for example NumLock is one of them.
At least I cannot prove that there is anything wrong with the ReactOS WM_MOUSEMOVE.
... but I'll have a look. (maybe I'll already find something useful in the bugzilla)