Ah, that helps! As does your diagnosis that FilterManager is causing the problem! 
Okay, first of all, a solution:
Set the priority of the shadow-casting light to a negative value.
But of course, and as indeed shown by the program you posted, you’ve tried that, and it doesn’t work. I confirm as much.
So set it to an even more negative value! XD;
(Specifically, I tried with the somewhat-ridiculous value of “-1000000”.)
On my end, this causes the problem to go away!
To speculate, what I think is happening is that FilterManager, naturally, wants to render the scene before the final rendering is done, in order that said final rendering be done with a current view of the scene.
To this end, I imagine that it’s somewhere setting the priority of the scene-rendering to a negative value.
As a result, if the priority of the shadow-casting light isn’t set to a sufficiently negative value, it still ends up having a higher priority than the scene, and so its shadow-buffer is still rendered after the scene, and thus we still get a one-frame delay in shadow-rendering!