Bedrock Dedicated Server external connections fail

Environment:
- Bedrock Dedicated Server 1.26.x
- Windows
- AMD Phenom II X2 545
- Public IPv4, no CGNAT

Symptoms:
- LAN clients connect successfully.
- External clients receive NetherNet / InitialConnection-13.
- Wireshark confirms UDP packets arrive at the host.
- Port forwarding and DMZ tested.
- Multiple clients tested (Egypt and Romania).

Observed behavior:
- UDP 19132 fails externally.
- UDP 19133 fails externally.
- UDP 19155 fails externally.
- UDP 8420 works externally.
- UDP 8096 works externally.

When using 8420/8096, clients can fully join and play normally.

The issue appears related to the transport layer or port-dependent behavior rather than basic connectivity.

Additional observation:
- On at least one BDS version (1.26.30.5), external connection attempts could trigger a crash with:
Exception code: 0xc000001d
Faulting application: bedrock_server.exe