The fact that it sort of works with the grounds connected should be a clue that maybe that is the correct way of doing it. When you specify a voltage you reference something, the default reference is ground. Electronics is the same way and also needs a reference. The output of your voltage divider is about 2.6V, on the sketchy side for a 3V3 processor and low for a 5V processor. You will not be able to measure that with a digital pin, the best you will get is on or off. To measue you need to use an analog input.
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