Should log out and login after the electricity is went out

i had one problem on spark. an example of this was i had 2 clients, client A and B. when there was no electricity the server ( openfire ) still turn on owing to UPS. when it was on client PCs, client A could see client B online but client B could’t not see client A online.

can anyone tell me why? does openfire automatically refresh online users to spark every 5 minutes??