I always leave ECON on. What I've found is that if the battery is fully charged and you initiate the regen paddle, the ICE will come on. Basically, the system recognizes that it cannot feed the battery any more charge and because the driver asked to slow the car with the regen, it decides to turn on the ICE to do that. The ICE is then reluctant to turn off again. I've had mixed success activating and then deactivating HV mode to get back to EV driving. I've been slow to remember not to use the regen paddle when the battery is full to avoid this situation.