Creality CR-10S Pro v2 bed temperatur drops

Good day,

Could someone advise on what the fault may be with my Creality CR-10S Pro v2.

Every print, whether a print of a couple of minutes or a couple of days, ALWAYS drops the bed temperature within the first quarter of the print.
I have to watch the print constantly, and as soon as it drops, I have to change it manually to the print’s original bed temperature. After that, it will maintain at the temperature I put in, even if the print still needs to go on for 3 days.
It is a random time in the print - I cannot work out the time into the print or the % into the print; it differs with each print, but is always in the first quarter of the print.
I have checked the G-code, and there is no command to drop the bed temperature.