Beekeeper
It’s a disease without a cure!
the reproduction sending units are terrible for accuracy simply because the rheostat winding in them is different than the originals at best they will tell you full and empty but in between is not accurate. I say the gauge is defective but you could ground the sending wire once again and see if it goes to full if so as soon as I gets there unground the sending unit wire immediately. But I think from what you stated the gauge is problematic at the very least.