Including a hash character (#) in a math equation causes it to not render (in new-question page)

Question

I want to ask a question about a paper that uses this notation:

However, when I typeset it in MathJax with $r(ξ, \bar{ξ}) = \frac{1}{\#𝒢} \sum_{g ∈ 𝒢} r_0 (g(ξ), g(\bar{ξ}))$, it displays the entire expression in unformatted plaintext surrounded by a box:

$$ r(ξ, \bar{ξ}) = \frac{1}{\#𝒢} \sum_{g ∈ 𝒢} r_0 (g(ξ), g(\bar{ξ})) $$

The problem seems to be the hash character. If I remove it, everything else renders correctly as normal. Confusingly, this exact equation renders perfectly fine on math.stackexchange.com, hash included, which means the problem is not universal to MathJax in general. The equation also displays fine in an external LaTeX renderer.

UPDATE: After posting this question, I saw that the equation renders normally in the posted page. It also renders normally in the edit-question page. Apparently, the problem only occurs in the new-question page.