When exporting a hosted feature service from ArcGIS Online (AGOL) to a File Geodatabase (GDB), some of the key GDB settings that you originally set up are lost in the export. I have encountered this when feature services were originally created via ArcMap "Share as a Service" from a File GDB. Please fix the AGOL export so that all of the original GDB settings are preserved in the downloaded File GDB.
Specific examples of settings that are lost:
1) "Default Values" for all attributes of the feature class and related tables
2) GDB "Domain Description" text
3) Feature class that you export has a different name in the download GDB than it was in the original GDB
4) Relationship Class(es) have different name(s) in the download GDB than in the original GDB
5) Relationship Class "Notification" and "Label" settings are removed