Game crashes - alliance wars

The game crashes everytime if I am in Alliance Wars and switch to a different app (e.g. line) and then straight back on to the game. I get the usual message that the game has disconnected. This does not happen when I am in other modes of the game e.g. Alliance Quest or normal quests.
Samsung S9 - mobile data 4g
Samsung S9 - mobile data 4g
0
Comments
iPhone 8, all connections, iOS 11.4.2