The Pug's ancestors did not have flat faces - While this breed originated in China, there are no documents proving his appearance at that point in history. However, if we look at the breed 300 years ago and what he looks like today, we can see a development over the past 3 centuries in which the face gradually became shortened.
This leads us to believe that ancestors certainly did not have a lack of snout if this was only a relatively recent change.
For this reason, a so-called Chinese Pug would not have a flat face. Pugs from several hundred years ago actually had notably longer snouts than the Pug that we know of today along with longer legs and a leaner physique. Paintings of Pugs began appearing in the late 1700's and early 1800's, most famously by the painter Francisco José de Goya y Lucientes.
To look into this further, we can note the remarkable painting from 1745 of English painter/sculptor William Hogarth (1697 – 1764) which shows his Pug named Trump to have a moderate snout and wide-chested yet lean body.
Paintings all throughout the 18th and 19th centuries show Pugs to have moderate snouts, tails
with just one curl, a wide chest, moderately long legs, small ears
and no apparent large, bulging eyes or flattened face and forehead. These traits, therefore, must have been perfected in Europe over the late 1800's and early 1900's.