We switched from ß to ss in all words with a preceding short vowel in 1996: de.wikipedia.org/…/Reform_der_deutschen_Rechtschr…
So, it’s “Fuß” and “Maß”, because those are pronounced with a long vowel, but then “Fass” and “muss” and “Biss”, because those are pronounced with a short vowel.
And in this case, “bisschen” is spelled with a small “b” for reasons that I’m not entirely sure are logical. 😅
It would be spelled with a capital letter, if “Bisschen” was a unit of measurement here (i.e. a small bite), like a “Liter” is.
But because it was used so much and without really referring to a specific measurement, it eventually began being spelled lowercase, similar to “wenig” or “etwas” (“ein wenig Deutsch”, “etwas Deutsch”). Apparently, this kind of word is called an “Indefinitpronomen”.