Thank you for reaching out. This is a limitation of the build system right now: for permission control, the build uses a minimal scoped token that only has access to the resources directly used in the build, and the build can only access a media reference if the media set itself is an input of the build. In your case, the build originating from “Simulated Materialization” only has access to the dataset but not the media set, which led to the error.
We are looking into updating the build infra to better handle pipelines like this. In the meantime, a workaround is to add the media set into the build by joining it with the dataset (the join does not need actually do anything: you can drop all rows from the media set before joining, and let the join simply keep all rows from the dataset).