I ran into such a problem in the game, I started a new game in a single-player game on a difficult mode, if I replenish the hunger and thirst level of a character by 100%, it no longer goes below 100% until I re-enter the game, after I re-enter the game everything works fine until I replenish the level again hunger and thirst at 100%. Please solve this problem. I play without mods.
When you fill your hunger and thirst meters to 100%, you get a 10 minute buff. After the 10 minute buff goes away you will start to get hungry and thirsty again.
Full hunger meter = You have additional heath and and don’t get hungry.
Full thirst meter = You have additional stamina and don’t get thirsty.
Hope this helps. BTW, welcome to the forum. 
Oh thank you very much now I will know about it
Thanks from me also after all this time I didn’t realize that 