Just wondering if this is happening to anyone else. In the more recent versions of Collector (definitely 20.1) on iOS, after I create an offline cache sometimes one layer will not show the features that I know are there. And if I try to create a new feature, it will also be invisible. But if I quit Collector and reopen it, voila the features are back. It's almost like the symbology is broken.
I have had a similar issue but on the AGOL web map and View side. The features are visible in Collector itself however. See my post for more info - https://community.esri.com/message/930257-offline-edits-not-appearing-in-hosted-views