It was a bug, if you just block someone the block might not work as planned.
It was, I'm not sure if it is anymore, that it's linked to the bio bug.
If you want to be sure someone doesn't see you, you need to make sure you are in the first map you joined, then you need to add them to your block list, and then rejoin once more in order for it to stay.
or else they will still be able to see you, the only thing that will happen is that you will loose sight of them.
So always rejoin after a block, and maybe block them once again after you get back just incase.