To be clear, you seem to be asking if anyone has had one or both problems
1.) A CEL related to an O2 reading (?)
2.) DEF level that does not read full after filling the DEF tank.
I have never had a CEL. I have had a non-responsive DEF tank level. In my case, I was doing one of those long days where the vehicle had not stoppped/nor was shut off well for over 750 miles. My DEF tank was running low so I stopped and added urea but the level never came up. I shut the truck off, let it sit for about 30 seconds and restarted. Level never came up. I continued to drive for another two hours and the level continued to drop until I arrived at a pilot car construction zone. This time, I had shut the truck off for 10 minutes while waiting for the pilot car. When I restarted, the level came up to the level I expected.
I do not know the algorithms for how the DEF tank reads level, but extended key-off reset periods seem to be necessary if you refill.
In your case, I'm not all that concerned about your DEF level. but your NOx sensor sounds like it is unhappy. I'd have the dealer work on that one first (assumes this is covered under powertrain warranty)