When attempting to write to an existing file geodatabase within a zip file, FME overwrites the contents of the feature class, and will not insert new data into the existing feature class.

Also, when I try to set the FeatureWriter's Table Handling mode to 'Use Existing' FME does not recognize that the table exists, and produces an error that the table does not exist.

If anyone would like to try this out, here are the steps to reproduce:

• Set up a workspace with a FeatureWriter and use Esri Geodatabase (File Geodb Open API)
• Set the FeatureWriter to create a geodatabase and zip the contents
• Run the workspace and write the data
• Re-run the workspace and write the data
• Inspect the results, and you'll see that FME has overwritten the previous data, and the feature class will only include the data you wrote when running the workspace for the second time.

Using FME Desktop 2021.1 (Build 21525) on Mac OS (11.2.1 Big Sur).