Solution? A strange bug from Joomla.
I deleted my overrides and I intervened directly in the original file:site/layouts/joomla/content/info_block/category.php
In this php file, I change the class css "category-name" to "category-nametest" .That doesn't work, it's always "category-name."
Crazy! It is directly on the source that should be effective!
I do this test again on a Joomla Simple: It works!
Well, then it's T4 that screws up?
I test on a Quickstart T4 (Stark) It works. So it's not T4!!
Of course, I refresh every time (empty the cache also in case)
In despair, I review my category display options on my menu. I deselect the display of categories, then I put it back, and, suddenly CA MARCHE!
So it would be an update that is not done automatically.
Note, on my blog page, I also did the test with "Written by" and I had no problems. It's really this category.php that's not updating well!