Hello olivier. majo
There are two things that need to be considered here:
1. Line weight and Linestyle that we assign to Layers apply only to 'Layout' lines/curves. So, you may want to set 'Layout' to 'True' in the properties. (refer below image)
2. To Display the Lineweight or Line style, you will need to enable the 'Lineweight' option from Display> Show. (refer image below)
Once those 2 conditions are ensured, you should see your Lineweight settings take effect to the respective layers as shown in the above image.
Is this what you are looking for or am I missing something? If so, can you please explain your issue in a more elaborate manner?