Atlassian have a bug where some browsers will refuse to insert the draw.io macro (or any other macro). A minority of installations are affected. There is a Jira ticket, please vote and complain on it. The reports mention Chrome and Safari, but never Firefox, IE or Edge.
draw.io for Confluence Cloud is a plugin for Atlassian's Confluence Cloud product.
draw.io provides embedded diagramming functionality to add a range of diagram types to Confluence pages. All diagram data is stored only in Atlassian's cloud, directly from your browser (it doesn't pass through our servers during save/load).