Operator precedence not respected

I want to define an operator >>= which performs member access (i.e. is equivalent to .).

My first attempt does not compile:

operator >>= left 19 = (left, right) => {
    return #`${left}.${right}]`;
};

My second attempt (using computed member access) works but not as expected:

operator >>= left 19 = (left, right) => {
  let r = right.name.token.value; // string representation of an identifier
  let dummy = #`dummy`.get(0);
  return #`${left} [${fromStringLiteral(dummy, r)}]`;
};

I deliberately give my >>= operator precedence 19, which is the same precedence as javascript's member access operator (see JS operator precedence).

Based on the given precedence i would expect this to be possible:

var obj = {
    foo: function() {
        // return a promise
    }
};

obj>>=foo.then(res => { console.log(res); })

However, this does not compile. For some reason the right argument of my >>= operator is a call expression (i.e. foo.then(...)).

So, based on the given precedence i would expect obj>>=foo.then(...) to be equivalent to (obj>>=foo).then(...), however it seems to be interpreted as obj>>=(foo.then(...)).

If i manually add the parentheses as follows (obj>>=foo).then(...), then it works.