Another one of those who is responsible issues so I'll dual post.
Was just looking at the changes made to support subtypes in AGOL. While it seems there was a valid attempt made in this regard it would appear to not actually work. My guess is that the issue is that in the ArcGIS Server service definition the json name used is 'subtype' while in AGOL it is 'subType'
So at 100.5 and current AGOL you can still not use FeatureSubtypes. The SubtypeField property is correct in both cases