Sounds as an asterisk chanspy feature/bug/misconfiguration. You will have to look at the asterisk full log with debug enabled in order to see more information. IN fop2 there is one setting that can drop your session, but it only does that if the spied on channel hangs up. To prevent that from happening, you can turn it off, in /usr/local/fop2/fop2.cfg
persistent_spy=1
(This setting will be no longer available in FOP 2.27, as the option to drop the session when the spied on channel ends is already implemented in recent asterisk versions).
Best regards,