Hello Pascal,
Thank you for reporting this issue.
For some clarification, here a gradient is used in order to have a solid hatch of a different colour to the boundary. It is difficult to manage the colours in the end because it is not linked to any layer and cannot be modified with a single click on a large file. The possibility of colouring the outline and the hatch separately would be simpler and would perhaps solve this problem too?
see similar related topics:
Hatch object including outline and solid background
Display Hatch Boundary
And maybe here a related problem:
Draw order broken for hatches