Retest your CYA. It was 50 a month ago. I find I lose about 10/month. It may be that you are in fact not at 50 any more. Because I lose 10 a month. I have a good understanding of my pool usage, and I actually start at 60 CYA and keep my FC at 7-9, and watch CYA drop to 50 and adjust my FC appropriately. I'm not saying that YOU should do that, but if you have a good understanding of the chemistry and your pool, you will learn how YOU want to manage it, and how it behaves.Right! It makes no sense to me. I hate to raise the cya in case I have to slam again at some point, but I feel it’s too risky not too if I’m losing this much to sunlight. Also, the LC I’m using is Pool Essentials from Walmart or Lowe’s. Is that a decent brand. It is not expired.
If you are at 40, the loss from 9.5 to 6 is not surprising, given 40 CYA will not buffer much above 6-7. If it was hot, sunny and you had swimmers, it easy to lose the other 2.5 (from 6 to 3.5) in a day. You and I both have large pools, large surface area. My 20x40 has 18K gallons in just the first 3' of the pool. Easy to lose FC with heat and sun and swimmers. I have a pool that is 1/2 shaded by 40 foot trees, and on an 80 degree day, sunny, swimmers, I lose 2.5-3. By your picture, you have nothing shading the pool.
Check your CYA.