Invalid rendering for `\sum` when it 's used as a numerator

[novak-as]

Separate \sum_{p=0}^P M_p^i is ok: $$\sum_{p=0}^P A_p$$

Combined with \over: $${ \sum_{p=0}^{P} A_p \over B}$$

Combined with \frac: $$\frac{ \sum_{p=0}^{P} A_p}{B}$$

I believe this is also could be related to https://github.com/github/markup/issues/1573