Shader graph confusion!

(LWRP using shader graph) I have a black hole shader which is using screen colour to give a lensing effect. It works perfectly in scene view, but refuses to co-operate in game view.

I have narrowed the problem down to something relating to the opaque texture on/off in the camera settings. I can assure you it is turned on in the render pipeline settings.

If i turn on opaque texture in the camera settings, i get a bad looking and glitched shader.

If i turn off opaque texture then i just get a grey sphere

however if i start out with opaque texture off in camera settings, and then - while the game is running - turn the opaque texture on, then off again, it starts to work perfectly in game view....

What the hell is going on and how do I make it work all the time?