Try booting your phone without the charging port connected, you do not need to remove the charging port, just unclip it from the board and then boot the phone. See if the problem persists, if it doesn't then the new charger flex is damaged. It's quite common for new charger flex to sometimes not fully work. We make a habit of ordering 2 charger flex for our iPhone repairs for this very reason. If the problem is still occurring after disconnecting the charger flex then yes there is probably a logic board issue :(
Good Luck :)