This happened to me yesterday. I had just gotten a low DEF warning so I was going to fill it, but the tank only took about half a box. Between 1 and 1.5 gallons. Checked the gauge and it was full. I drove it for another 100 miles and got the 500 mile warning again, except this time it went from 500, to 300, to 100, to 60, to 10, to limp mode in about 30 feet. Took it straight to the dealer and of course, they turn it back on and it says 100% full.
It's at the dealer now but like with the "crank no start issue", they've never heard about this and I'm sure they're going to find nothing wrong.