Including a theme icon in Confluence 7.0.1 and up won't work

Issue Summary

When developing a Confluence theme by following https://developer.atlassian.com/server/confluence/adding-a-theme-icon/ and adding a custom icon, the icon won't be displayed in the admin theme configuration screen. Instead, a default theme icon is displayed.

Environment

Confluence 7.0.1 and up.

Steps to Reproduce

1. Create a Confluence theme and add an icon resource to it as per the instructions on the page above
2. Navigate to the admin theme configuration screen

Expected Results

The custom configured icon should appear.

Actual Results

The default theme icon is displayed instead.

Workaround

No workaround at the moment.