FYI it's a bug (BUG-000121105).
As far as I can tell this was introduced with the 3/19 AGOL update (tech could not confirm, but I didnt have this issue exporting on 3/18). I'm not too pleased with Esri at the moment as this is the 3rd recent issue I'm facing directly related to updates.
This is particularly egregious because we (meaning the entire Esri user community) have no other way to back up data from ArcGIS online.