Calculate field based on another joined field fails

[deleted-user-jxpbpbjlZInj]

I'm working in ArcGIS Pro 2.2 and I have a feature class from a geodatabase called "Seating." There is an empty field called "TSP_UID." I have joined another table from my local drive to this table.

Next, I wanted to calculate the seating.TSP_UID from the joinedtable.TSP_UID. 

In ArcGIS Pro - calculate field fails and gives me a 999999 error. In ArcMap - it says calculate field is successful, but it doesn't actually alter any of the attributes. 

Here's the python snippet from ArcMap:

# Replace a layer/table view name with a path to a dataset (which can be a layer file) or create the layer/table view within the script
# The following inputs are layers or table views: "TDEC_GIS_TSP.Seating"
arcpy.CalculateField_management(in_table="TDEC_GIS_TSP.Seating", field="TDEC_GIS_TSP.Seating.TSP_UID", expression="(!TDEC_GIS_TSP_JoinedTable.TSP_UID!)", expression_type="PYTHON", code_block="")‍‍‍‍‍‍‍‍‍‍‍‍

UPDATE: So I closed everything out and opened ArcGIS Pro back up and MAGICALLY the field in question was suddenly populated?! So - now this is even more annoying because I can't figure out when this occurred or why it finally worked, and I've got a handful of other feature classes to do this with. Yayyyyyy. ESRI MAGIC.

[KoryKramer]

Sunny, if you give us a few more details about what consistently causes an Error 999999 (i.e. what is the data type of the target field, data type of source field, if Text, what is the character length, etc.) it would be good for us to take a look.

Thanks

[deleted-user-jxpbpbjlZInj]

Hey Kory - Sorry for the late reply! I'm afraid I may not longer be helpful because it mysteriously began working. What I experienced was the above, but then I started trying another way and it would then mysteriously show the fields populated as they should have been if the above had not "failed"... i.e. it was showing that it failed, my field values remained null, I'd run another process and suddenly they would show up like the original process had run properly. 

I HAVE NO IDEA WHAT HAPPENED! 

My TSP_UID field that I was trying to populate is Text, Length 8. 

I was trying to populate from a joined excel spreadsheet.