For the sake of helping others with similar problems.
The UDP (game data) protocol is incorrectly implemented in Toxiic's emulator.
The game client sends connect packets to the game data (UDP 5350) and a STUN-server (UDP 5351) to detect the NAT-type of the local network. If the STUN-check detects symmetric NAT, then the client will start sending packets to the server so it can relay these packets to the others in the game room. Toxiic's emulator does not properly handle the STUN-check to detect the NAT-type nor relaying the packets to peers. Some players are unable to send the packets to the other peers causing them to be invisible.
A solution is to enable NAT in the router (if possibe) or re-implement the game data handler and STUN-check.