Here is the problem you are facing. What is happening in your pool is pretty much not possible.
There is no secret ingredient or magic trick to solve this problem based on what we all know.
FC is consumed by two things only.......sunlight and organics in your pool......there is nothing else that consumes chlorine in a pool.
So, by performing the OCLT, sunlight is eliminated but you still pass the test confirming there are no appreciable organics in your pool.
Sunlight can be the only culprit. However, even with NO CYA whatsoever you would not consume 20 ppm in a morning....perhaps 10-15.
Couple that with your confirmed reading (and dosing) of 40 ppm CYA, that means your AM loss might be 5 ppm, if that.
It's a scenario where there is no answer.
OK, so I'll re-try the OCLT. The drastic consumption is real and is only happening during the day. And the CYA is really there because I watched myself put it in and have confirmed the test enough to need to order more R-0013. Its just funny that if there are organics in my pool, they only consume chlorine when there is daylight. At night or when the cover is closed, I'm holding very steady to the 0.5.