I have the same issue. It's like, my chest flattens down and then upwards, giving the weird cleavage. I've found wearing baggy shirts helps hide this a bit better. I also have to be careful not to slouch forward too much or bend down (I would rather squat down then bend) as the tshirt collar falls forward and can give onlookers an eyeful of awkward cleavage...!
I haven't found anything to fix this as I am already wearing the binder that compresses the best without affecting my ability to breathe and isn't impossible to get on and off...
Sorry I couldn't help you much!