Artisan sourdough bread
As moscafj mentioned in comments, the baker likely changed something in the process or recipe.  If the general question is why big holes happen in lean artisan breads sometimes, there are several possibilities, some of which I enumerated in answer to a previous question.

In very general terms, the more bread is "handled" during the process of a rise, the more even the crumb will be.  (That is: "even crumb" = very fine small even holes.)  This can involve additional kneading or folding or "punching down" periodically during the rise.  The most relevant, of course, is handling and shaping before the final rise where the final air bubbles develop.

The baker is correct that many "artisan bakers" value a texture that has large (often irregular) holes.  And many consumers like that.  It may not be great for sandwiches, but that is not the only use for artisan bread.  If, however, the holes are not evenly distributed but are gathered in certain places (like the middle of the top of the loaf, as you mention in the question), that could indicate inferior kneading or shaping technique, as well as perhaps an overproofed final dough before baking.