Mathjax bug

f(\sum^\infty_{n=1}b_n)=
\begin{array} 
f(\sum^n_{i=1}a_i) \\ 
\sum^\infty_{n=1}f(b_n)=\sum^n_{i=1}f(a_i)+f(0)=\sum^n_{i=1}f(a_i)
\end{array}

produces:

$$f(\sum^\infty_{n=1}b_n)= \begin{array} f(\sum^n_{i=1}a_i) \\ \sum^\infty_{n=1}f(b_n)=\sum^n_{i=1}f(a_i)+f(0)=\sum^n_{i=1}f(a_i) \end{array}$$

f(\sum^\infty_{n=1}b_n)=
\begin{array} 
fg(\sum^n_{i=1}a_i) \\ 
\sum^\infty_{n=1}f(b_n)=\sum^n_{i=1}f(a_i)+f(0)=\sum^n_{i=1}f(a_i)
\end{array}

produces:

$$f(\sum^\infty_{n=1}b_n)= \begin{array} fg(\sum^n_{i=1}a_i) \\ \sum^\infty_{n=1}f(b_n)=\sum^n_{i=1}f(a_i)+f(0)=\sum^n_{i=1}f(a_i) \end{array}$$