Confluence Cloud has a bug that sometimes does not update the diagram revision in the macro, even though the call to saveMacro returns success. We have tried to workaround the issue, but it's not technically possible. Atlassian need to fix it, their issue is at https://jira.atlassian.com/browse/CONFCLOUD-58848. Please send your displeasure in their direction.
Detecting the condition has not been possible to date, since the state the macro ends up in looks the same as if a user reverts to a version of the page that references the last but one version of the diagram. UPDATE March 2018, we're looking into a workaround that takes the date stamps into account.
This means a diagram is saved, but the macro linking the diagram to the page isn't saved, so no diagram appears.
The quickest workaround for now is to download the diagram attachment from the attachment section of the page:
Go into page edit mode and insert a new draw.io diagram into the page:
When the draw.io editor appears, drag the attachment you saved into the editor canvas. The diagrams will appear. Next, save the diagram and save the page.
We're actively investigating how to workaround this issue.