Per the documentation this variable is defined as follows.
Specify whether to hide the property by default from the Performance Center user interface. If this tag is set to "true," you can manually display the custom attribute in the user interface. This tag impacts whether custom attributes appear by default in the Performance Center user interface only. They are available by default in other areas, such as the OpenAPI QueryBuilder.
Note: If this tag is set to "true", custom attributes are hidden from context pages and cannot be manually shown in the user interface. If you want custom attributes to appear on context pages, you must set this tag to "false."
Update the Hidden variable for any Custom Attribute involved to use false instead of true so it appears as follows: