Nope, only bonne chance is correct. Is it possible you’re thinking of bon courage?

As you said chance is a feminine noun, so you say bonne chance. However french people tend to pronounce “bon'” (a short “n” with no e) and not “bonne” (long ‘n’ with ‘e’ pronounced), maybe that explains your confusion?

There is only one correct way: bonne chance.

The n-gram you provide was queried against the English corpus, not the French one. There is no “bon chance” on the French one.

If you’ve seen it written “bon chance”, it’s possible that it’s been misspelled by the writer, because bon can be pronounced like “bonne” in front of masculine word starting with a vowel, like in “bon appétit”. But it’s just a mistake.

The only right way to say it is: bonne chance

You got that false answer with bon chance because your corpus is set to English. If you set it to French, you will get the right and unique answer:

“Bon chance” isn’t valid in French for the reason you wrote in your question. Chance being a feminine noun, the proper spelling is “bonne chance”.

Though, “Bon chance” is said as a “joke” (at least, in Quebec, don’t know about the other french speaking places). It’s a reference from the movie Taken. I don’t know if this is universal, but this is where most people I hear say “Bon chance” got it from. (It happens at the end of the phone call between the kidnapper and Liam Neeson in the first movie)

I’m a french native speaker but I often use “Bon Chance” as a gimmick with an “Eastern-Europe Accent”…

That’s because of “Marco de Tropoja” in “Taken”…

BUT, of course, the right way to spell/say/write/ear/understand it is “Bonne chance”.

It should be bonne chance because chance is feminine so bon should also be feminine. bon is masc, bonne is feminine.

chance is feminine, so it should be bonne.

Bon is for masculine nouns, such as this:

Bon lit, Bon garçon, etc.

They are both masculine nouns thus using bon would be correct.