From what I've read, GLP-1s tend to level off after about 60 weeks. Actually, most weight loss methods become less effective the closer you get to your ideal weight.
As you slim down, your metabolism slows because you don't need as many calories. Your body adapts to exercise and conserves energy in other ways, like reducing muscle building.
The cool thing about GLP-1s is that 60 weeks is way longer than what you get with just diet and exercise. So, you have a longer window to lose the weight you want.
It's not really immunity; your body just fights harder to prevent further weight loss as you approach a healthy weight. And sometimes, your body's idea of a good weight differs from yours.