Which type of Hunter rain sensor do you have? Can you post the model number? I think it has to do with either the way this particular sensor works, or there may be a loose connection somewhere.
If it’s a simple normally closed sensor, when it detects rain the two wires on the rain sensor should remain open-circuit. You can use a multimeter to verify this. Normally open sensors are the reverse way. If the multimeter shows that it connects and disconnects and repeats like that, then the sensor will not work with OpenSprinkler.