ST1S14PHR power instability

[lbug.1]

I'm using ST1S14PHR like the attached schematic, but with an input between about 10V and 20V the output is instable and appears 2.8/3.2V ...sometimes after a few seconds the stable situation at 5V is reached.

After the ST1S there is a linear 3v3 regulator that with the instable input has an instable output.

Absorption at 24V is about 30mA.

What could be the problem?

thanks...