It seems not to be possible to terminate rtt with:
cameraeffect ["terminate", "back", "rtt"];
Instead you need to use it like this, that terminates all current rtt's:
cameraeffect ["terminate", "back"];
That forces to reset all current PiPs if wanting to close one PiP. Still works nice but the limitation is giving much extra trouble as seen in this vid:
Using "rendertarget4" to "rendertarget8"