I put together a VTK viewer in Python. I also extended QVTKWidget to host the viewer in a Qt app. It all looks and works fine with Qt 5.9.7 and VTK 8.1.2 on windows (in a conda environment).
If I set up the “same” (Qt, Python, VTK versions are the same) environment on Linux, the QVTKWidget renders on its own window outside the Qt application so the interaction which is passed from Qt to VTK happens on the Qt window, while the display happens on the OpenGL2 window opened by VTK.
I have seen this behaviour before and I believe the fix I found was to change the VTK version. Is that something that you recognise?