*** UPDATE: Atlassian confirmed that this is a bug in Jira and opened issue ECO-738 ***
Thanks a lot for your feedback. Unfortunately, the (modal) project page editing experience has some technical limitations from the Atlassian Cloud side that breaks 3rd party macros. This limitation does not only affect our app, but also apps from other vendors. As soon as Atlassian addresses this limitation, editing and inserting will work. Right now, you have to edit the pages in Confluence. The (modal) view mode is *not* affected, i.e. you can edit/create the pages in Confluence and you view the pages (including the Mermaid diagrams) in Jira's modal view.
By the way: Do you know that our app is also available for Jira (https://marketplace.atlassian.com/apps/1224537/) which allows you to add diagrams to Jira issues?