Incorrect math inline rendering `$\sum_{j=1}^n$`.

[mlondschien]

This is an inline $\sum_{j=1}^n j = \frac{n (n-1)}{2}$ expression.

renders as

This is an inline $\sum_{j=1}^n j = \frac{n (n-1)}{2}$ expression.

[mlondschien]

Possibly related to #1579.