Hey gods! (@CraigFeldspar and @sebavan)
I have found another problem using prepass renderer (or at least using prepass renderer in SSAO2): the overlays and bouding boxes are not rendered anymore once the SSAO2 post-process is created (using the prepass renderer). I’m checking if layer effects (glow layer etc.) are not affected, will let you know 
You can repro here: https://playground.babylonjs.com/#PQKG24#3
Which works pretty well using legacy buffer renderer: https://playground.babylonjs.com/#PQKG24#4
Thanks again for your help!
Edit1: the playground is still using rc.1 but I have tested locally with rc.3 and I still have the problem
Edit2: if the object with overlay has its material flagged “disable depth write = true” then it works well