In Collector for ArcGIS (Android), when modifying an existing feature from a non-null value to a null value for a nullable coded domain value field and submitting the edit, the edit does not get saved and the previous value remains for the feature.
Issue was non-reproducible with Collector for ArcGIS v17.0.2 for Android devices.
解决办法
Follow the same workflow in Collector for ArcGIS on iOS. The field editor for the nullable coded domain field includes an option in the picklist for "No value" which saves a null value upon edit submission.