An AI whose training model only rewarded images that mimicked 19th-century art would never develop a "style" outside of that. But if you had a model that was trained on 19th-century art, and then provided a critic network that rewarded 20th-century art (as well as 19th), and re-trained the network, I suspect that it would develop a 20th-century "style", as variations that previously would have been rejected under a strict 19th-century criteria will now be accepted.

Contrary, I wouldn't expect 19th century artists locked in their rooms to produce a new cohesive art style. You cannot even get artists to reliably create 19th century art at scale. They quickly get sidetracked, and keep complaining about the bars on the windows.

Art evolved in relation to the social and historical circumstances. At the very least you would need to give all the context of the 20th century beforehand (except for the art itself).