I poured some bleach on a rag and started scrubbing.
The rag got really hot after 30 seconds.
Just to make sure it wasn’t the friction between the rag and the toilet seat, I took the rag, set it on the counter, and poured bleach on one part of it.
Sure enough, the part I poured bleach on was hot to the touch, unlike the rest of the rag.
The rag was part of a pair of old jeans.
Is there something in jeans that reacts with bleach?