We can take a look into this; in the meantime try explicitly adding GRAPH <tag:stardog:api:context:default> around the triples you want inserted into the default graph.
So it is read-only. My apologies! I have created a ticket for us to look further into this issue, as it appears that even in your first case it doesn't quite work as expected. In my testing both of the triples were put into their respective graphs, but also into the default graph.