Items within a graphics view get partially drawn outside the view on the remainder of the main application window, nasty. So far it doesn't seem to cause any crashes, but it makes the application completely unusable, this issue should have high priority. Also, this problem does not manifest in Qt 5.6 (binary distribution from your website).
The following C++ code reproduces the problem: shows an ellipse drawn in a graphics view, and the ellipse is partly drawn outside the view! As long as there is more than one proxy widget in the scene, the bug appears:
Setting the opacity of label_item to just below 1 (like 0.9999) gets rid of the problem
The fact that this is a regression, and that opacity < 1 works fine, likely make this bug really easy to figure out, but I'm not familiar enough with Qt to track it down and I don't have access to a C++ dev env here (I use Qt from Python only). I bet that some different logic gets executed when opacity is 1, as an optimization because that's the most common case, whereas opacity < 1 requires some blending which has a cost and hence associated code not run when opacity = 1.
This bug is probably related to 55070 but I wanted to produce a separate bug report since it is different code, and really this should have higher than P3 priority given the impact and the ease with which it is likely to be resolved.