This is a great observation and hopefully my answer will be clear enough to understand.
Way back when warfare was man-to-man combat, High School Dressage training was the key to unlocking the potential in a horse for war. The close combat required horses to be highly collected, and carry most of the weight on the hind legs in order to stay in place, while still being able to maneuver around obstacles. It really didn’t matter how “beautiful” the horses flat gaits were and it certainly was not a concern when the horses hind end and neck were extremely well muscled, but the mid back looked dropped.
The goal was a light, responsive and obedient horse that would help the rider in battle to win, and the most important factor was that the horse was 100% capable of collection. People were breeding horses for collection and these were those baroque type horses. Antoine de Pluvinel was the man that really spread this idea that horses could have more willingness and lightness to do its job for humans.
Until the French Revolution started, the horses were muscular, round, and able to collect very easily.
With the French Revolution and cannons and gun warfare brought an end to the collected horse and the art of the war horse. This was because it took up to 12 years to school a horse in the highest level for war, but it took one second to shoot that horse down with a gun. There was no more motivation to take all this time to school horses when they would surely be killed.
Instead of schooling 100 horses to the haute ecole, Napoleon wanted thousands of fast horses of the Arabian and thoroughbred blood who pushed with their hind legs. If 100 of the horses were shot down, there would be 900 more to win the battle.
Back to your question:
It really depends on the trainer and the method. Some trainers follow the old Renaissance style (now called Academic Art of Riding) and in this style, there is not much emphasis placed on how “hollow” the horses back appears. How do you get a horses back to not be hollow? you push them forward. This is not the goal of the Academic Art of Riding. The goal was to make the hind legs carry more weight.
It also depends on the horse. Trainers like Bend Branderup and Marijke de Jong have had baroque horses who have an easier time carrying weight, and warmblood and hot breeds who tend to push forward more with the hind legs. For a trainer who emphasizes lightness and self carriage, depending on the willingness of the horse, the trainer might not want to push the horse forward in an attempt to raise the back.
Here we can see Bend Branderup with his horse preforming a rather hollow piaffe:
and here we see him with a different horse preforming a very engaged passage:
In academic art of riding, collection and lightness are more important then avoiding this “hollowness”.
Some trainers follow a different path of “classical”. Here you can see that to Anja Beran, lightless comes first.
(take note that I’m not passing this up as a correct passage. Im just explaining that in the French school, lightness comes before accuracy)
Conclusion: There are many different pathways of Classical riding. Each has its own set of goals and ideals. For all of the reputable “classical” trainers, I have come across good photos, and some not so ideal photos. The best part about the classical art of riding, is that the horses and riders are always improving. The art is a self motivated one. You don’t see these trainers competing. In sport dressage, when you reach the highest level and still preform crappy, hollow movements and you get high 90s for test scores and the fans go wild, is there still room for improvement?