My issue is that when Im selecting (masking) a part, it actually selects an another part than Im selecting, when the image is flopped.
I dont know how to explain it better.
Try to flop an image and apply and feather a mask and you will see what I mean.
Right that has to do with the after effects order of operations, which you can't change. But if your mask is in the precomp and your flop effect is in the main comp you should be good. When you need to modifiy your mask, you need to go into the precomp.