Does your TV have discrete power on and power off? When you setup your TV with the wizard what "power options" did you specify? Depending on if specifiied "one button powers on and off", "separate buttons to power on and power off", etc that might be the problem. If you specified the "separate buttons" option but selected the same "Power" (toggle) button then Harmony will thinks it's okay to always send the power command since, if the TV is already on sending "Power On" is a no-op. However, in the event that the "Power On" is actually a "Power toggle" then the Harmony doesn't know this and it will power off the TV.

If this is the case and your TV does not have discrete power buttons, then change the TV settings to "one buttons powers on and off".

You can go into the device menu for the TV and press the "Power On" button while the TV is already on to confirm whether the "Power On" is actually a "Power toggle". Some devices have multiple "Power" buttons.