The reported behavior is expected when the add method is used. The Scheduler component treats the newly added events as temporary ones until they are synced. To achieve this, the datasource should be configured with its transport option and a "create" endpoint should be added.
Talking about the data method, it generally changes the dataSource of the component. This is why an event is not disappearing when added using this method.
What I can suggest as an alternative to the "add" method is to use pushCreate instead. This Dojo example demonstrates how the mentioned method can be used.
Progress is here for your business, like always. Read more
about the measures we are taking to ensure business continuity and help fight the COVID-19 pandemic.
Our thoughts here at Progress are with those affected by the outbreak.