TPS54231: Non-liner output response

I use TPS54231 for two buck switcher designs  ：5V-to-1.2V and 5V-to-1.0V .There have same problems.

Below is the output. Green is 5V.Yellow is ouput.Pink is the SS pin.Blue is the PH pin.

Below is the schematic.

Step1 ：I noticed the SS capacitor is too big.But the article on the forum told that is ok. I change the capacitor to 22nf. The problem remains.

Step2 ：I added the 1 Meg resistor in parallel with the SS capacitor(0.1uf). The problem remains.

If the chip boots up repeatedly and frequently, It will reduce a large overshot.

I wonder if there is a reasonable explanation. 

Thanks.