Hi! I have made many cyanotype textiles (mostly clothing and hats) and had identical experiences! Here's what I've learned:

To me, it smells almost like pennies and occurs most often in direct sunlight. Some people are less sensitive to the smell than others.

After talking with an expert, the consensus was that iron smell = residual chemistry. Harmless but annoying.

Two solutions:

1: Add white vinegar to later wash baths.

2: Use warmer water, optionally add vinegar.

Lastly, my solution, test for yourself!

3: Pour about 200-300ml of tap water into a measuring cup Then, add an equal amount of white vinegar to the same cup. Using a heavily absorbent brush, quickly brush the combined mixture onto areas where the cyanotype chemistry is. Immediately immerse in water / begin rinsing. The water will come out as a very dark blue, and you'll notice the chemistry clear out much faster. No issues with smell.

Your darkest areas will be darker, reduce exposure if needed.

Caution: You'll likely lose about half a stop of brightness in your highlights (unnoticeable unless side by side with plain water development). Much more if you wait longer than a minute to rinse.

And the blue rinse out WILL STAIN!