Hi Everyone,
I contacted ESRI support on this issue. It is a known bug in ArcGIS Desktop 10.6/10.6.1. Below is the response from ESRI tech support.
I uninstalled 10.6 and reinstalled 10.5.1 and the export files work perfectly with the right block objects and attributes visible in AutoCAD.
ESRI says that they support up to AutoCAD version 2017 with format (dwg) version 2013. I created the seed file in AutoCAD 2018 and saved to .dwg format 2013. The ArcGIS output file opens fine in 2018 and 2019. I haven't created any seed files in AutoCAD 2019, but it seems like it would work as long as you save to .dwg 2013 format.
Ben
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Response from ESRI Tech Support
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As I mentioned in my voicemail message, I have taken ownership of this case and I am very sorry that you are running into BUG-000112999 that was logged to report that Export to CAD does not create BLOCK entities properly, along with attributes in the output CAD file at version 10.6 of ArcGIS Desktop.
This process did work correctly in ArcGIS Desktop 10.5/10.5.1, and allows exporting to AutoCAD DWG format through 2017. That is the latest version of AutoCAD DWG or DXF format files that is supported in ArcGIS Desktop. AutoCAD files through 2017 are supported in ArcGIS Desktop 10.5 and 10.6. In order to run this Export to CAD process, using a seed file, the seed file must be the same version as that selected for the output DWG or DXF file, or the process will not produce correct results.
Here is a link to the supported CAD formats from ArcGIS Desktop 10.6/10.6.1:
http://desktop.arcgis.com/en/arcmap/latest/manage-data/cad/supported-cad-formats-in-arcgis.htm
The supported formats for 10.5/10.5.1 are slightly less extensive:
http://desktop.arcgis.com/en/arcmap/10.5/manage-data/cad/supported-cad-formats-in-arcgis.htm
AutoDesk Corp. does not share their libraries with Esri. Esri purchases libraries to read and write AutoCAD files from a 3rd party vendor, then incorprorates those files into our software. This is of course a time consuming process, and explains why Esri does not yet support AutoCAD r2018 DWG or DXF file formats.