After a lot of research, couldn't find a fix for this problem.
An alternative was to find another firmware, wich led me to the famous Open DSO 150. After flashing it, all measurings are now correct (wich proves that the original firmware is defective). As a bonus, I have other options to play with the DSO, I recommend this ongoing work!
Attached is the .hex file, wich should be flashed as the original JYE Tech, like it is explained in the
tutorial. This file is the pure compilation of
michar71 work, just converted to .hex, wich can be flashed with an USB to UART TTL converter. Remember to hold the "OK" buttom on the first start up to reset all values to defaults.
Remember, do it if you know whats happening, I'm not responsible for any damage

Credits to
michar71:
https://github.com/michar71/Open-DSO-150