I was wrong ;) This is not only a matter of check box it seems a bit more complicated... Your guess was right it is because of the canvas however when I set it to "Scale with screen size" the text becomes crispy. I saw on the internet that it was possible to do things differently with anchors. I'll try check that tomorrow morning it is currently 1 a.m here in France and I did not completely recover from the jam ;)
Also I figured out that my UI was made only for full HD resolution (1920x1080) I am wondering what screen you have... Whatever thanks for the review I appreciate !