Clock frequency reduced about 1 second after configuration

I synthesized the blinking led example given here.
When I download the bitstream in SRAM the design works during about 1 seconds as expected. But after this time, the frequency is reduced and the design works about 10 times slower.

Is it a known bug ?

Nobody have similar problem ?
I tryed with new design (in chisel) that I publied here. Platform pinout is given here.

And I probed the pin XI of CH552 (pin 17). I can measure correct frequency during about 1-2 seconds then the quartz shutdown suddenly.

Is there a bug in CH552 ? Does anybody else have similar problem ?