Why is the release burnup chart showing too little done work?

This is probably because the line for done work only includes backlog items that are completely done (that is, have status Done or above). It doesn't take into account work in progress on backlog items that are not yet done. This is different from the burndown chart for a sprint, where the work in progress of a backlog item is also included. So, a release burnup chart is a bit more conservative than a sprint burndown chart.