I have been having a problem with gray spots on my whites and light colors for years and cannot figure out the cause. I have changed washers, had my water checked, have different pipes installed, and always separate colors from whites, but I still get gray spots. They almost look like redeposited dirt. I thought it could be a hair product, but the spots are not always in the same area. They are all over. The spots are not on the clothes until after I wash them, and then I cannot remove the stains. I have tried using bleach and color catcher sheets and that has not worked either. Have you ever heard of this problem?
This a tough one without seeing the specific items. I admire all the legwork and changes that you’ve tried. All were potential problems and eliminating them is definitely a plus for you. Also, you have good laundry practices with the sorting, etc.
I agree that it has to be something that is being redeposited. So a couple of thoughts:
If you are using fabric softener, it can also react with carried over detergent forming a paste-like substance that can deposit on clothes. This can be removed by treating with bar soap (wet, rub in) and then rewashing.