I am running into this same problem (if you look back through old posts, it has actually been many years since this issue first arose)
Upon looking through other GeoNet posts and troubleshooting documentation on exporting tables from AGOL, I have concluded that there does not seem to be a good work around. Subtype and domain descriptions are unreliable and frequently lost when exported from AGOL to any other data table, geodatabase, or feature class file format.
This topic has been an ongoing issue for years...Does anyone have any advice? Why hasn't ESRI fixed this, or at least provided some very straight forward documentation about the limitation in exporting subtype and domain descriptions from AGOL?
(Other than exporting into a FGDB; this method does not work for me)
Any advice is greatly appreciated!