Thanks to you I have a solution !
First, you definitely need to check that you work with the same color space settings, both in Affinity Designer and in Affinity Publisher, or else the colors won't appear the same when you print your document.
Second, when you want to insert a vector drawing made with Affinity Designer inside an Affinity Publisher document, the best solution is to group all the curves and/or shapes of your vector drawing in Affinity Designer, then copy this group and paste it into your Affinity Publisher document. This is the only way to keep your illustration vectorized. If you use the place image tool, or file > place in Affinity Publisher, your vector drawing will look rasterized.
I made different tests with my black and white laser printer and I can tell this works.
That's too bad the image tool, or file > place doesn't work very well with vector designs, but you have come up with a great workaround !
Thank you very much for you help !
Lanval