Hello Patchi,
Sorry for the trouble. I checked for some indications preventing to merge into a single object. But I did not found clear mention of it on the wiki. I thought the way to cross the intersection was obvious (turning anticlockwise in France), but I forgot that not only cars use roundabout. I thought also it was easier for routes relations to have only one object (quite small at the scale of the entire route) rather than many parts that could mess up the route if somebody would splits parts incorrectly (I was first editing routes and that what I found in some of them).

With the linked discussion I understand the issue. I will revert my work. Thank you.

Meinew