#41 may be related.

Describe the bug:
When a fraction with aligned arguments is displayed with KaTeX, the bar is not centred vertically between the numerator and the denominator, in contrast to LaTeX.

(La)TeX code:
The code of (La)TeX you tried to render:

\frac
{\begin{aligned}(M)\end{aligned}}
{\begin{aligned}(N)\end{aligned}}

Expected behavior:
I would expect the numerator to be lower.

Screenshots:

Environment (please complete the following information):

• KaTeX Version: Editor at https://katex.org, display mode
• Device: Desktop
• OS: macOS
• Browser: Firefox | Safari
• Version: 87.0 | 14.0.3

Additional context:
When using \frac to display inference rules, where both the premises and the conclusion may be multi-line, the extra space above the bar looks (slightly) wrong.