Why do French nasalized vowels take a "u" in English?

Why do French nasalized vowels take a "u" in English?

In my last post I noticed that English words borrowed from such French words as are now pronounced with nasalized vowels often add or added a "u" to their spelling. So "enchaunt" and "romaunt" had "u"s although we now spell them without: "enchant", "romance". The "u" has dropped out after the "a" in the spelling.

But after "o" this "u" has remained, and the vowel is pronounced /au/: Compare "round" with French "rond", "counsel" with "counseil", "soun[d]" with "son". This is the same pattern that Katherine seems to have noticed when she blanched at the pseudo-English "coun" (for "gown") because it sounded like French "con".

But Katherine was not noticing a pattern: she was put off by an audible similarity in the pronunciation of the two words. Obviously /ko˜/ and /kaun/ have very little phonetic similarity nowadays. So how did they sound in the past? And does the past pronunciation explain why these words have a "u" in English? I have no idea.