Here are a few things to check:
1) Did you remove the old AVR (Meridian) from your Harmony setup? If not, the Harmony will send the power command to turn it off. If the old AVR (Meridian) used the discrete PowerOff command, this also means your new AVR (VSX-53) understands that command even if it's not in its dataset.

2) Review the activity and check the list of actions. Does it list an unforeseen command (that you may have added using the activity Start/Exit custom actions feature).

3) A command for another device is also interpreted as a Power Toggle or Power Off command by your AVR. If this is the case, then you will need to use a different dataset (Mode, IR address, etc.) for your new AVR, or for the offending device. Not all devices support multiple addresses, but most high-end AVR do.