Using the “semicolon” key with a space before it does not work. Remove the space and it does.
\starttext
\setupmathematics[autopunctuation={all, semicolon}]
\m{a;b}\quad \m{a,b}\quad\m{a.b}\quad\m{1;11}\quad \m{1,11}\quad\m{1.11}
\setupmathematics[autopunctuation={all,semicolon}]
\m{a;b}\quad \m{a,b}\quad\m{a.b}\quad\m{1;11}\quad \m{1,11}\quad\m{1.11}
\stoptext
Alan