Your belief about which is the "correct" pronunciation depends on whether you believe the "correct" etymology is French or Italian. It appears to me that people agree that the word came into English from French earlier, but also that (at least in 'murica) people have settled on the other pronunciation.
It's not that there's a correct pronunciation that got corrupted, as you suggest. There are two legitimate derivations of this word with different pronunciations and one has won out.
By the way, the word in French would be pronounced \ˈfȯt\, which is a common usage in Britain. And it refers not to a fort (which is the Italian etymology for "a strong point") but rather to the part of a sword between the middle and the hilt.
Source: I like to read about usage and etymology. Sadly my dictionary of classical word origins suggests that forte comes from fortis in Latin, which (while ultimately true) doesn't reflect either later usage or pronunciation.