Error 000728 when trying to calculate field with joined hosted feature layer in AGO

[KellyWright]

I have a hosted feature layer of fire hydrants with a related hydrant maintenance table living in ArcGIS Online. I need to be able to add the Asset ID from the hydrant HFL to a "Hydrant ID" field in the related table. In ArcGIS Pro I joined the two tables and tried to use Calculate Field to copy the asset ID from the parent table to the related records. I am getting an "Error 000728: Field <value> does not exist within table." The field definitely exists. If I export the join to a separate feature class in a file GDB, I am able to complete the operation using Field Calculator. (trying to join this feature class to the hosted maintenance table and field calculate the data causes the error again)

Why would this not work with a hosted feature layer, but work in a geodatabase? I've read other posts online about this error, but none of them deal with hosted feature layers. 

Any advice is appreciated. I am at a loss.

[Robert_LeClair · Accepted solution]

My understanding of joins is that you can calculate an attribute field in the parent table but not the child table.  Meaning the non-spatial table that is joined to the spatial table is considered read-only.  When you export the joined tables to a new feature class, then the join is removed and you have one large spatial table upon which you can do field calculations.

[KellyWright]

Thank you @Robert_LeClair! This was the correct answer; as soon as I did the join in the opposite direction (from the related table to the feature layer) it worked with no issue. Much appreciated!