Other Fuel System Tank Filling/charging problems of the 2015 Ford F-250

I was heading from cibolo, TX to kingsville, TX. (distance 181 miles) upon arriving at destination I shut down my truck. After approximately one hour, I started my truck and the check engine light came on. I connected my obd reader and it identified the fault code p21b0 (I did not clear any codes). From my research it relates to the �reductant level sensor �c� circuit high�. After researching more information I started looking through the information center under the vehicle setting and noticed the def level was below half (I never got a pop up or notification from the information cluster). In the past the information cluster would display the level notification without an engine light coming on. This time it did not. I purchased some def and filled the tank. It only took two gallons of def. The engine light remained on the rest of the day driving around after filling the def tank and the def �below half� notification also remained on. The next morning I started the truck and the engine light was off and the def �below half� notification was gone.