Firmware 2.1.6 auto-refreshes the RF signals to make sure the outlets are in sync with the controller. So if your outlets do not have separate on/off switch (which means it only uses a toggle switch), or if you are manually switching them on/off in addition to using the OpenSprinkler firmware, it would cause the issue you described.
The temporary work-around is to revert back to firmware 2.1.5.