Having max/min spare servers as the same amount is a bad idea. This is incurs a substantial amount of process swapping, as every single request php-fpm is going to try and ensure there are precisely 256 idle servers.
What happens now is 256 workers running and 256 simultaneous requests occur. So php-fpm sees 256 workers busy, 0 idle. The minimum idle is 256, so it attempts to start 256 additional processes.