Hello - I'm having a similar issue.
A default value is set in ArcGIS Pro, the default value shows as such in AGOL, but when adding a point with Field Maps there's no default value showing and leaving it blank results in '-' being collected as opposed to the default value. Any ideas?
- I've tried removing and adding the default value in Pro and overwriting the web layer
- I can't add a default value in the Data properties of the web feature class