Hi Jean-Michel,
The short answer is that pagination adds an extra wrapping div to your output, and so that new div is your grid item instead of the actual posts. See this thread for solutions.
Your wrapper CSS should probably be changed to look like this:
Thanks a lot Julia.
By the way, I read the post you mention before trying this code and could not make your own code to work (very frustrating). I don’t know why. It may be interesting to write in the documentation an example of CSS for a simple grid rendering for noobs like me.