Can not log in to a server again once exit

I own a server with official 1.12 jar. When I go into the server first time, it connects well. When I got the app in backgroud (like making a phone call), it will disconnect from server but the server keeps me in. When I back to the game, the server shows like  [Player:1/10] but I'm already out and I cannot enter the server before I reboot it.