I did a quick run on the Training Grounds. It looks like, as others mentioned, the counter stops at 10 but still triggers at 15.
+250 damage AND major blaze damage may just be a little confusing. I suspect it's 250 and separately some additional blaze damage, the distinction would be important to people that get into specifics with builds. Keep in mind, dealing enough of the given elements damage will light a behemoth on fire, so that matters too.
In my test, excluding crits, I was getting 250/204 part/wound damage, Hit 15 was 592/481 just as an example of the differences I saw, core got the same numbers as part damage. (you'll want to hit slow and on the same part to make sure it's uniform if you're curious)
In short, it seems to be working but there is a visual issue with the icon. Definitely keep using it if you like it though as it is functioning.