Rare connection issue: server responds to query but ignores connection packets (VPN or restarting router fixes it)

[Joey251]

Describe the bug

A rare connection issue where some players can see the server in the SA-MP client and the server responds to query packets, but when they try to join they receive: The server didn't respond. Retrying...

This affects a very small percentage of players (approximately 2 out of 100). The issue appears to happen after a server restart. Packet capture shows that the server responds to query packets from the player but does not respond at all to connection/handshake packets. Interestingly, if the player connects through a VPN, the connection works immediately. When I restart the server, the players who previously couldn’t connect are able to join, but then some other players start experiencing the same issue and can’t connect anymore.

To Reproduce
The issue is difficult to reproduce consistently, but the observed behavior is:

1. The server appears online in the SA-MP client and responds to query packets.
2. A player attempts to join the server normally.
3. The player tries to connect.
4. They recieve message: The server didn't respond. Retrying...

Expected behavior
//

Screenshots and/or logs
(packets with size 4 are connection packets, while the others are query packets from same player)

Release version
v1.5.8.3079