Hi,
I was able to append the data with the addFeatures endpoint in the REST API, but I left the question open because I thought more information on this process in AGOL would be helpful in any case.
Here is the entire process that I did.
I retrieved the layer in question (which was originally uploaded to AGOL and updated with Field Maps) with the REST API (endpoint query) and inserted it in PostgreSQL with PostGIS enabled. I then drew some new features (polygons) while connected to this table in QGIS. I did not use ArcGIS Pro.
I tried then converting the new features (rows) to a zipped shapefile and a geojson, and both did not give a perfect match when trying to append as shown in the capture in my original post.
I have not tested this method on other datasets.
Concerning your question : Does the dialogue give you the ability to manually match the fields? The answer is no and that is definitely a big problem! I could not reassign the Match Field to other field or select a field for the ones that had no match. I guess if I could have changed this, I would not have posted this question 🙂
Thank you!