Why the SSD work in SATA 2 mode only?

I bought Chuwi Lapbook Pro CWI530 8G DDR4 / Netac 256 SSD. The SSD in my laptop only works in SATA 2 mode though it can work in SATA 3 mode. I have a bios E.G140J.D8.E1.015. On one Russian forum, people say that who has the previous version of the BIOS, the SSD works in SATA 3 mode and tests show the speed is almost twice as high.
Some guy downgrade BIOS from E.G140J.D8.E1.015 for previous version E.G140J.D8.E1.006 (from https://www.mediafire.com/folder/77em5bdgfgs6c/Bios) and it start SSD in SATA 3 mode.
Is it bug? Why newest BIOS not support SATA 3 mode for SSD?