This can happen if Control Center can only "toggle" the power of your AVR/sound bar (as opposed to being able to specifically tell it to turn on or off). It is possible for devices to get out of sync if a they miss the toggle command from Control Center. To fix this, when your TV is on, simply power the AVR/soundbar on manually using the device front panel or original device remote. This will put all the devices in sync (power wise). After this, your TV and AVR/sound bar should power on and off together when controlled from the Control Center remote.
To help prevent this problem in the future, you may want to use an IR emitter placed directly in front of the device's IR window. This will reduce the chance of a power command from Control Center being missed by the device. Control Center ships with a dual IR emitter that connects to Control Center via the 3.5mm IR port.
Also, if your AVR/sound bar is connected to Control Center via HDMI, try enabling HDMI-CEC on your device (if the device supports it). HDMI-CEC will enable Control Center to track the power state of the AVR/sound bar. After you have enabled it, reset, and re-configure the AVR/sound bar by going to Settings > Devices > [AVR/sound bar] and selecting "Reset Output".
