Hello everyone,
Thank you for reporting the issue here! This is an interesting topic!
Although the custom expressions are visible in schedules, Find & Select and Text tool, it has been a design decision that some of the Properties cannot be used in Graphic Override rules. The reasons behind it are partly technical, partly the performance. For example: properties that cannot be processed in the background, or the calculation based on the model are not available.
In this case, since the custom expression (Area of the window) is a calculation based on the settings of the window placed on the wall, Graphics Override automatically hides the expression.
The wish to have this feature to be implemented has been recorded in our Wishlist database (please refer to it as #12116). I have updated it, hopefully, our Product Management team will adopt this one in the future.
Thank you all for your contribution, and I wish you a great day!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT