Hi @frankderuyck,
I did the same steps as you and have the same design problem, with randomization in the whole plots for the "Hard to Change" factor.
However, with the Custom design, I'm able to have a design that do respect the restriction on randomization of hard-to-change factor X5 :
I would use the "Custom Design" platform for this kind of use case, as this seems more accurate and relevant (a split-plot design might not be an "easy" DoE ).
All the best,
Victor GUILLER
L'Oréal Data & Analytics
"It is not unusual for a well-designed experiment to analyze itself" (Box, Hunter and Hunter)