Append data to hosted feature layer seems to process normally (success message), but no new data gets added to the layer.
This workflow was operational a few days ago. I am using a zipped shape file.
AGOL Append is still not operating properly, but a work-around using ArcMap was successful:
1) make a local copy for editing,
2) then append in ArcMap,
3) then synchronizing back to AGOL
AGOL seems intermittent, as I had to repeat step 1 several times until data came through to ArcMap (10.5.1); zero rows otherwise.