I believe this is an Obsidian issue but it might be a Mathjax one, in which case I suppose it’s out of your hands.
When rendering Latex math in the preview mode, symbols that get stretched very tall have the middle part not line up with the ends, making it look quite bad. It’s hard to describe, easier with a picture.
This latex:
$\mathbb{R}^n = \left\{ \begin{pmatrix} x_1 \\ \vdots \\ x_n \end{pmatrix} \middle| x_1, x_2, \ldots, x_n \in \mathbb{R} \right\}$
produces this in the preview: 
Notice the weird skinny section on the parentheses and curly braces; the part of the symbol that gets stretched doesn’t match the rest of the symbol