I had a long and boring but kinda scientific explanation for this here's the better one. (since proteins don't do anything in-game i won't mention them)
You have an apple - 100 calories - (tiny amounts of fat + tiny amounts of carbonhydrates)
You have an ice cream - 2500 or more calories - (lots of carbonhydrates + lots of fat)
You ate an apple and it satisfied your hunger for 1 - 2 h
You ate an ice cream and it satisfied your hunger for 2 - 3 h
but here's the catch you got 2400 + calories more then you did from an apple and it only satisfied your hunger for an hour.
Now all of that excesive amount of calories that you took will make you gain a layer of fat, and eating high calories foods for breakfast then starving yourself over the day is the case for you. Nature's way is not a bug, you expirience this every day in real life and by doing excercise of any kind can help you with this calorie problem, ofcourse the best solution for you is to eat fruits and vegetables, try making a fruits or vegetable salad and only eat 1/4 of it (15 is enough to remove the first hunger moodle) if you do this, and continue killing zombies i will guarantee that you will lose 1kg every 2-4 in-game days, but you have to do exacly as i said, you know what will happn if you stuff down excesive amounts of salad right?
Yeah I've noticed this too when i was running away from a goup of zobmies in Riverside, they followed me from that mall to the parkings lots above the hardware store and i was runing in a straight line + never broke their line of sight, also sneaking levels and trait inconspicuous don't do anything / not as nearly as before, I'm unable to play sprinters anymore because some zombies spot me from distances larger then our ability to zoom out, by far these are the two most annoying (possibly bugs) additions that weren't here in b41.50