I have no issue with Larian home brewing some new monsters. But if they're going to do that, they should just create new monsters. Phase spiders are iconic and, while many people haven't encountered them before, those who have are going to expect them to behave a certain way. It's not the end of the world to have your expectations confounded now and then, but the point of using icons is for quick recognition and understanding - don't mess with them.
