Modal dialogs belonging to subapps that are configured as the first Subapp of an app do not appear.
This can be tested with the About App on the demo.

The "jira" subapp of the About App contains a modal. In the configurations, it is currently the last subapp on the list, but if you put this as the first subapp, the modal will not appear if you open the About App.

If you switch tabs then go back to the Jira tab, then the dialog will appear.

For apps with multiple subapps, this isnt too much of a problem because you can just move the subapp's node to a different position on the configurations, or switch tabs. But for apps who only have one subapp, this becomes a problem.

Steps to Reproduce

• Go to https://demoauthor.magnolia-cms.com/.magnolia/admincentral
• Open Configuration -> modules -> about-app -> apps -> about -> subApps
• Move "jira" to the first position
• Open "About Magnolia" app.
• As result you see the empty page, dialog to enter Jira credentials doesn't appear. no modal dialog appears

Workaround

• Click on a different subapp tab then switch back to the subapp with the modal and the modal dialog will appear.

• In the case of apps with only one subapp, create a "dummy" subapp so that you can either switch between tabs in order for the modal to appear, or set it as the 2nd subapp on the configurations.

Other Notes
This started happening in 6.2.15 and above.