This might be a synchronization problem between the server and the client. Would you share your server with Dropbox for further investigation. Server route: ~\Planet Explorers\Server\ServerData\CommonGames（or CustomGames)\(Server Name)
We've fixed this bug for next update, but if you've already get this bug, you need to restart to fix this. As this one is a sideline quest, you can also ignore it and continue to play with the mainline tasks.
The reason why shield seems of no use when facing multiple monsters is that too many monsters attack at the same time will lead to attack damage offset in vain. It's not because the defence is out of work.