When adding it as an item (and publishing as a hosted feature layer) I was also unable to share the layer with everyone (nor was it displaying in any map viewer). However, when I added the item to a new project and published it as a hosted feature layer, I was able to successfully view it and share with everyone. Maybe it has to do with how you are adding the item? Are you just adding it as an item straight to your portal? Maybe publishing can be a viable workaround?
Using Enterprise 10.8.1 with latest version of Pro.