Steps to reproduce:
In user preference, turn on Export mathemaical formulas as MathJax
Type \multline*
to create a multline environment
In the menu, click
Edit » Copy to html
The result is
<center>
\(\displaystyle \begin{array}{l}
\sum_{k = 0}^{\infty} (\alpha + k)
(\alpha + k - 1) a_k x^{\alpha + k} -
\sum_{k = 0}^{\infty} (\alpha + k)
(\alpha + k - 1) a_k x^{\alpha + k - 1}\\
+ \sum_{k = 0}^{\infty} 3
(\alpha + k) a_k x^{\alpha + k} + \sum_{k =
0}^{\infty} a_k x^{\alpha + k}
= 0
\end{array}\)
</center>
multline
environment becomes array
environment, so the second line is left-aligned (but should be right-aligned).