The way that FCM gathers received bytes and delivers them to the Data Handler in “blocks” is a very common cause of questions:
https://forum.sierrawireless.com/t/whats-a-block/399/6
https://forum.sierrawireless.com/t/rs-232-performance/5077/2
https://forum.sierrawireless.com/t/q2686-uart-receive-buffer-size-is-limit-only-56/4436/10
https://forum.sierrawireless.com/t/fcm-and-wip/4407/10
https://forum.sierrawireless.com/t/q2686-uart-buffer-size/5031/2