One more question, what's it look like on your hosted feature layer's settings page under Feature Layer (hosted)? Do you have any of the first 4 check boxed checked concerning editing?
I tried replicating your situation:
- I created a polygon feature class in a file geodatabase in Pro. Manually added a EditDate and CreationDate fields.
- Published to AGO. When I checked "Enable editing", no fields were added. When I checked "Keep track of created and updated features" and "Keep track of who created and last updated features", it finally added the "automatic" fields, seen in yellow. Since I already had fields named CreationDate and EditDate, it named the new fields "_1" at the end.
- When I edited in AGO, only the yellow fields automatically updated. This is because AGO knew these fields were to track created/edited features. The ones I manually made in Pro, it knew nothing about so it didn't autopopulate them. They just happened to be named "EditDate" and "CreationDate" but didn't have the intelligence behind them.
So if your fields aren't autopopulating, I'm thinking they were manually created fields in Pro that happened to be called the same things as ArcGIS looks for. Since you don't have duplicates of these so named fields, your "Keep track of created and updated features" and "Keep track of who created and last updated features" may not be checked on. Not sure if you did this since I'm not sure if they're autopopulating correctly or not, but best practice (for Pro) is to "Enable Editor Tracking" on a feature class like this (see below). Otherwise, like I just mentioned, it can be done in ArcGIS Online. This will add all the needed fields and ArcGIS will know those are the ones it should be populating with the automatic values.
Still not sure about the default value on the date field, I would try to change that to None if possible.