BR Phantom layer visible only in renders - Particle Render

OK, I've spent days ( all week !!! ) messing around trying to fix this. 3D comps, text camera shared across all 3D merges.

comp is 6200X2400 24 fps

the problem is I have a phantom layer edge visible ONLY in renders. In normal comp windows, HQ / draft, MB On / Off etc its generally not visible when working with comp. problem 98% of the time only shows up in renders - HQ full res or 1/2 res draft, doesn't matter.

I've done all the usual things - check all nodes for good image sizing, made nodes / layers larger than comp to not go off edges, etc. Even checked camera for near / far clip DoF off, everything is good. Yet this phantom layer remains. at the moment my only conclusion is this is a OGL render error in Fusion 18.6.5 & 19b1 that only happens during rendering.

Wait I take that back. if I set the comp view to 100 or 101% the bad layer becomes visible, never at smaller size. Then it's gone. This bug has been maddening for the amount of time it's consumed.

Ok after shutting nodes off yet again trying to isolate the problem, I maybe finally isolated it to pRender output.
Trying to send pRender to cache but lovely bug - the cache output from pRender via loader node can't be sent back into the next nodes. but pRender outputs something that isn't Fusion RaW format. Perfect(ly wrong ) . Who designed this cache ? its got other bugs as well like not using cache files but render again at render output, and even deleting the cache at render time even though it shouldn't. Ok, add a dummy merge, cache that, move output into separate folder to F can't delete it, manually load it. ok. start turning nodes / layers back on and so far ok

moment of truth : render comp even though its missing elements, its 80% at this point. I FINALLY HAVE A GOOD RENDER ! all WEEK wasted trying to track this down. I still have to add back some bg layer elements but the problem is finally fixed. it should not be this hard.