Appending features from a file geodatabase to a hosted feature layer in ArcGIS Online fails if the 'Preserve Global IDs' checkbox is checked in ArcGIS Pro 3.4.3.
This issue has been addressed in ArcGIS Pro 3.5. The 'Optimize performance for feature services' parameter for compatible feature services preserves the input Global ID values from file geodatabases when the 'Matching Fields for Update' parameter is set to the GlobalID fields. Refer to https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/append.htm#:~:text=The%20Optimize%20performance,Global%20ID%20values for more information.