Actually, now that I've seen the arms system, I honestly think that the way of implementation (I don't mean that it's buggy) but in general, I think that they should neither follow player's movements nor move on their own (this doesn't include the case if the boss itself moves).
The arms don't follow the player anymore, nor do they extend (they now have a fixed length). In the future (maybe the next small patch?) I'm thinking of resetting angles and length every time the attack is finished.
This way, the boss can execute more special moves while preventing the spawners from going to impossible places.