A mosaic dataset does not display NoData pixels properly on a ArcGIS Runtime SDK for .NET map when it is loaded as a mobile mosaic dataset (.geodatabase). This field can only be updated from the Detail page, and clicking the Functional Map Values button.
This appears to have been an issue in the creation of the mosaic dataset: the no data value was either not defined or incorrectly defined as 256. After the NoData value on the mosaic dataset was set to 255 to match the source raster data, then the NoData was honored in the ArcGIS Runtime SDK for .NET when displaying the raster data from a mobile mosaic dataset.