I don’t use scenes much, but I believe brightness_pct is not an actual attribute. brightness is (0…255). So you cannot use the (calculated) brightness_pct in scenes. If you put in brightness it will work.
I would have liked it to be an actual attrubute though. I have input helpers in percentages that I use to set brightness_pct. But I cannot use in state conditions to compare for them because I need to convert them to brightness first.