I have seen that happening during simulation, but usually it clears up once the simulation is done. I had a similar issue, but I was able to fix it with some moving around. Unfortunately I don't know what I did to get there.

Have you experimented with some extremes? Moving the button a bit in to deliberately cause more pokethrough, or pull it out further from the model, just to see what happens during the simulation?

If the whole waistband is constrained, it's sometimes easier to simply make the beltloops and possibly even the button constrained as well.