cisco pix 525 and packet filter

an linux server behind cisco pix 525 want to connect to a Freebsd
server behind NetBSD packet filter.

the client send syn packet with mss 1460 , it through pix 525 and pix
525 change mss to 1380

the FreeBSD server receive syn and syn back with mss 1460

the client received syn and retun ack, but the ack packet can't
throuth pix 525, then connect lose.

does anyone know why?

thanks .