I use a setup similar to what I described to fill a pond when the water gets low using a dry contact switch on a float and a hall effect flow meter, in the 3 years it has been runnning I never had the flow meter stop in a open position. I honestly hadn’t even thought about it till now.
I may have to rethink that setup, but I’m also intrigued by the fact it hadn’t happened. I wonder if the magnetic pickup causes it to stop in a certain position every time. I’ll tear in to it since winter is on its way in and I wont need to fill the pond.
As far as the rain sensor masking the flow sensor I don’t think it would be a problem since rain would inherently prevent flow, we would only loose the ability to detect leaks when it is raining.