IPv6 fragmentation remains a subtle yet impactful security concern in modern high-throughput, low-latency networks, where packet inspection is constrained by performance requirements and out-of-path monitoring architectures. This thesis investigates how discrepancies in IPv6 fragment reassembly behaviour between passive Intrusion Detection Systems (IDS) and endpoint hosts can lead to detection gaps, misinterpretation, and even exploitable evasion opportunities. Using a permutation-based testing model inspired by prior work, the study evaluates 720 overlapping fragment sequences across multiple operating systems and analyses the detection behaviour of Suricata deployed inside a 5G-simulated User Plane Function. The results reveal inconsistencies in reassembly policies, particularly under retransmission conditions, and demonstrate that alerts are not always semantically aligned with the payload seen by the host. A proof-of-concept exfiltration attack further illustrates how timing-based fragment delivery can bypass IDS inspection while reconstructing sensitive data at an attacker-controlled receiver. To mitigate these risks, the thesis proposes temporal IP ID tracking, overlap enforcement, and targeted traffic normalisation, especially in programmable or inline network functions. These findings highlight fragmentation as a persistent and under-addressed attack surface and call for more context-aware, timing-resilient detection strategies in next-generation networks.