We are running ArcGIS Pro 2.4.0 and have been experiencing similar issues. When working with a hosted feature service offline and attempting to perform a split, the operation will fail.
In our case there is an additional issue where, any edits that were recently made will then be lost from the offline geodatabase. If those edits were already synced to the service then they will be retained online, but never show up in the offline geodatabase until you disconnect and redownload. If the edits were not yet synced to the service then they will be permanently lost.
This has happened with multiple projects/services/users.