First of all, thank you for your interest.
I was using Affinity Designer Windows Customer Beta - version 1.9.0.820. I updated version 1.9.0.834. But it did not work correctly, the program was closing, I uninstalled it, I installed the 1.9.0.820 version again.
When I export without giving any color to the background, it appears like this.
A similar thing had happened a year ago. After a little review, I don't remember what I did in RGB settings.
I don't know what happened now.
My export settings are exactly as you show.
Thank you.