FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
The related article "Technical Note: Source NAT port range has been changed on FortiOS firmware versions 4.2.9 and 4.3.2 (and later)" explains that a source port will be translated within the range from 5117 to 65532.
However, the following session info shows source info "188.8.131.52 UDP port1" is translated to "172.20.0.1 UDP port 512". This port "512" is apparently out of the range:
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.