[reg 5.11->5.12] Windows: No more mouse click events sent to host application

I'm developping a Qt plugin (VST3) for a non-Qt host application (Studio One).

Everything was working fine until Qt 5.11.2.
As soon as I upgraded to Qt 5.12 beta 2/3, mouse click events stopped to be sent to the host application (here, Studio One).
This does not happen with other VST3 host applications (such as Steinberg VST3PluginTestHost or Cockos Reaper), but it's definitely blocking such events in Studio One.

As a result, while the Qt plugin still receive mouse clicks, no more mouse clicks are received by Studio One elements. Other events such as mouse move or keyboard events are still received. Tracing back the problem to a very minimal VST3 plugin, this happen as soon as a QGuiApplication object is created on Windows. Doesn't happen with a QCoreApplication object only, and doesn't happen on macOs.

I suspect this is related to the new mouse event handling in Qt 5.12 ?