Hello,
The really big bug is probably not in fop2 but in the asterisk version you have and using native attendant transfers on sip phones to a parking extension. FOP2 interprets the AMI commands, if it gets an AMI command of a channel being destroyed or a call being unparked, it will remove the call from the parking lot. The most probable cause is an incorrect AMI event of a channel being destroyed after the sip native transfer to the parking lot.
Try doing the same but using Asterisk native transfers, and not your phone transfer. You will see that it will probably work as expected. Maybe it will also be fixed if you update asterisk to a recent version, only way to know is by testing. I do not have aastra nor cisco phones myself to try, and I do not make test with every aterisk version, but with Asterisk 1.4.30 and using native asterisk transfers it works perfectly well.
Best regards,