Inspiron 3520 does not keep Boot sequence - Changes by itselg from UEFI to Legacy

Hello,

I have a dell Inspiron 3520 and i face a very strange problem. It is 2 years old and lately it does this strange thing. The boot settings should be on UEFI mode to work and boot normally. But even if the battery is fully charged and i suppose the CMOS battery is good since is 2 years old, when i shutdown the laptop and leave it for a minute...at the next power on it does not boor and in the bios settings at the boot settings, the boot mode changes from UEFI to Legacy by it self. I change it to UEFI, save the changes and boot normal. If I shutdown the laptop for a minute, at the next power on it does the same change in the boot optopns, from UEFI to Legacy. All other options and also Date and Time are ok, they never change. Only in the Boot menu is changing by itself....Firts time i see something like that. I have update the bios to the latest version, A10, but nothing changed, still makes the same thing.

Any suggestions please? Thank you in advance

Kind Regards,

Nicolas