We are running into a similar problem when trying to reload previously collected survey data to a republished Survey123 hosted Feature Layer. We're using Survey123 as our collector app for HazMat incidents (because the built-in Reports in Survey123 are pretty snazzy), and we have repeats built in for responding agencies, vehicles and chemicals involved, and for including photos of the incidents and the clean-up ops.
Using Append through ArcGIS Pro seems to work great for reloading all of our data to the republished survey EXCEPT the photo attachments. It creates the same number of placeholders for the photos in the appended data, but the actual photo attachments are left behind.
For example:
If report number 18-0807-1000 originally had 7 photo attachments in the repeat table (accessible and visible from the hosted Feature Layer's "Show Related Records" in AGOL Map Viewer, as seen in screenshot below)...
...the new Feature Layer with the appended data (exported from the original survey as a FGDB then appended in ArcGIS Pro) would have the same number of records in the related photo repeat table, but no "Photos and Files" to view (as illustrated in screenshot below).
The way it was explained to me:
"In short, as of now, there is no easy way to append features with repeats to a new feature class/service, regardless of a schema change. This is because repeats consist of a relationship with a related table based on a global ID, and the Append tool regenerates the global ID for each record when it is appended to the target layer. Essentially, when the new global ID is generated, the relationship to the repeat table is broken."
Makes sense, I suppose.
As of now, my understanding is that ENH-000113718 (Allow Workflow or ability for Survey123 to load, or append all records from a previous survey that included Repeats, i.e relationships in the Hosted Feature Layer) is currently marked as Not in Product Plan.
This is the suggested workaround: https://community.esri.com/groups/survey123/blog/2018/07/17/how-to-turn-supportsapplyeditswithgloabl...
The second enhancement, ENH-000113717 (Add ability or workflow that would allow Append operations to preserve Global ID for hosted feature layers with relationships), is In Product Plan.
If anyone has luck with this workaround, please share your scenario and let us know how it worked out. :-]