Learn English – Why the letter “g” discrepancy between *giant* and *gigantic*

Per the Naval Historical Center:

The English borrowed the word "sergeant" from the French in about the Thirteenth Century. They spelled it several different ways and pronounced it both as SARgent and SERgeant. The latter was closer to the French pronunciation. The SARgeant pronunciation became the most popular, however, so that when the Nineteenth Century dictionary writers agreed that the word should be spelled "sergeant" they could not change the popular pronunciation. Thus, we say SARgeant while the French and others say SERgeant.

Is your question: why is the "b" pronounced in "subtil" in French? The reason is that people in France started pronouncing "subtil" the way it was spelled. People have a tendency to start pronouncing things the way they are spelled. This is why the English pronounce the "h" in "herb" and the "l" in "solder". The Americans still pronounce these words the way they were pronounced in the 1600s.

Is your question: why is the "b" pronounced in "subtle" in English? It's not.

Is your question: why was there a "b" in the spelling of "subtle" in English and French? It had lost the "b" in the pronunciation in both countries several centuries before. This is because there was a movement—I believe in the 16th and 17th centuries—in both France and England to put letters that had been in Latin back into the spelling of words. Before this, the words had been spelled the way they were pronounced. An example of this is the word "partial". The OED says that in Middle French and Middle English, it was most often spelled with a "c" (e.g., parcial, although there were many variants). This is because it was pronounced with an "s". Looking at the OED's citations, the spelling with a "t" starts becoming common in English the late 16th century. It's still pronounced with an "s" in France, and the English pronunciation has evolved to an "sh".