OK, I just figured out the trick I did...
I'm using "user census" display mode with an alternative layout. But the only way I found to make it work with my alternative layout, was to hardcode/change the layout pass on the line 237 of mod_comprofileronline.php :
from :
require JModuleHelper::getLayoutPath( 'mod_comprofileronline', '_census' );
to :
require JModuleHelper::getLayoutPath( 'mod_comprofileronline', 'MyCustomLayout' );
Is there a way to apply alternative layout to census display mode without hardcoding the new pass ? I didnt found how to..