Bleach turned towel bright pink before fading to white
Although Dr. Laundry says it only happens to the incoming brighteners within a detergent and not to those fixed to the fabric, I think that your new towels should have enough brighteners - deposited into and/or onto them - to react as well.

Optical brightners and bleach reaction:  https://www.clorox.com/dr-laundry/detergentbleach-reaction/

Additional note : brighteners absorb light in the near UV and emit in the blu tail of the Vis spectrum, so making the fabric "extrawhite". Search for Optical brighteners in Google or Wikipedia.

Perahps someone here could know or propose the ongoing reaction and is transient behaviour. As the OP said in a comment, the towels were washed with a detergent containing a stilbenedisulfonic triazine derivative(s).