Hi, thanks for your answer.
Well, I find the previewing tool quite useful and effective to check color separation, if not for spot colors unfortunately.
I recreated the example page in the stable version (1.4.x), and the previewing tool behaves the same, that is : I can tick the spot color, but it has no effect on the preview.
The "convert spot color" option is not checked (neither in the document's color options, nor the previewing tool).
Thanks for the ghostscript tip, I will keep that for future use. But for this project, I need to send a pdf file to the printer.
In fact, I think everything is ok with my document regarding the spot color, it's hopefully just a bug with the previewing tool.
Now that I think about it, last time I worked on that document about 2 yers ago, I'm 100% sure I could preview the spot color.
Anyway, here's a sample page attached just in case.