When changing a feature's subtypes in agol, the behavior should mimic that of arcmap - all the default values of the newly selected subtype should be populated. This is vital for data integrity. This behavior works as expected in Collector (tested on iOS) but does not seem to work in agol webmaps on desktop.
The current workaround I've found here Problem: Changing subtype fields does not populate the expected fields in ArcGIS Online requires the user to either delete and then create, or to select the appropriate default values themselves. The former option is clunky and time consuming and the latter adds unneeded risk to data integrity and negates the purpose of designing subtypes to begin with.