Hi Achille,
Thank you very much for the question!
We once got a report about this issue on AC23, but due to the lack of reproduction method, we couldn't find out what triggered this. What we know is that, by resetting the Background color with this
guide, the background will be reset correctly.
Please let me know if this helps with the issue!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT