Ok so I answer to myself, after having tried to install the same configuration on local at home, I’ve found the plugin actually works perfectly. So I decided to desactivate my other plugins one by one in testing each time if I had the same trouble on my page titles.
And I’ve finally found that this plugin is not compatible with another plugin called “WP Image Carousel”.
When I desactivate this plugin, my titles are displaying like it should do.. So now, I just need to find another carousel ??
I’ve installed another diaporama to be sure and everything works good. I imagine if I’ve had 100 plugins to desactivate one by one to check which one makes the mess on my site. Anywway, what a waste of time :/