That's physic's telling you where you need to have your butt based on your current power output and position you maintain on the bike via all the different geometry and measurements involved. Changing the seat post won't cure that since it seems like the position you have set up on the bike for yourself is telling you to move the saddle back.

Unless I'm misunderstanding what you described. If you want your butt to stay forward then you need to increase your reach to the bars/aerobars and maybe increase your saddle to bar/aerobar drop to a lower position.