Problems with modem mode

Why IPhones have a problem with a modem mode ? After you lock the screen even for few seconds, the connection is disappearing, and after this cannot find the modem Iphone in Wifi list anymore, and need ask the person to reopen modem mode many times or change password or sometimes both and sometimes works sometimes not and need to wait very long and reload both phones.... When the company will already solved this problem, this is a terrible flaw?