Dark stains on white hand towels

I have white 100% cotton hand towels that all have a stubborn gray/tan stain that develops on them. In the photos, the folded towel is freshly washed and the unfolded one was used for a few days.

I wash my white towels together in cold water on the delicate cycle with OxiClean White Revive and Tide powdered detergent. My washing machine is front loading. Laundering them seems to reduce but not remove the stain, and as soon as I bring them back into circulation the stain darkens again. I do not wear foundation/concealer/etc.

What’s causing this stain and how can I get rid of it?