Disappearing Fuel Level

I have exactly 5 gal in the tank, poured from a VP fuel jug. With the cluster not touching the dash and the circled ring terminal connected the gauge read about a third of a tank. 50’ish ohms on the sender wire seems correct. The gauge reads zero with no power connected.

Once the cluster is installed or the second ground added the gauge goes to zero. I wouldn’t think additional grounds would be a problem. Maybe a short?