I've never owned a turbo version, so power mode / light is a new one for me.
It sounds like the car is complaining that there is a transmission issue when you are in power mode.
Most of the other Subaru "flashing lights" indicate a problem with the system enabled or monitored.
Your owners manual likely has a notation for this.
It might be a good idea not to use power mode if it is having a problem.
As far as what the cause could be... Good question.
You might want to see if there is anything logged in the ECU..