I received an email notification that Esri has discovered a bug in ArcGIS Pro 1.0. It appears that when using the field calculator in ArcGIS Pro it can cause certain fields to be set to null. This seems to occur when you calculate a field associated with a web feature layer or feature service. (These would be data sources you access via ArcGIS Online, ArcGIS Server or ArcGIS Portal) If you are working with a selection, it will only affect the selected records. This issue does not appear to be a problem if you are working with local data sources such as file geodatabases, enterprise geodatabases, or shapefiles.
Esri is working on a fix. In the meantime they recommend you do the following as a work around:
For more information about this issue, see Knowledge Base article KB 44521
Tripp Corbin, MCP, CFM, GISP | Chief Executive Officer
eGIS Associates, Inc.
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Esri Certified Desktop Associate | Esri Certified Enterprise System Design Associate