I have noticed that MathJax formulas are sometimes hidden behind other text and not properly displayed. I observed this only when using HTML-CSS as MathJax Renderer. (For other options the same formulas were displayed fine.) I am on Windows, using Google Chrome Version 78.0.3904.70 (Official Build) (64-bit).
Maybe there are also other factors, but I have seen this in centered formulas that contained \text{...}. I also did some minor experiments with such formulas in the Formatting Sandbox.
Here are some examples, together with screenshots how the formulas are displayed to me:
$$a=\frac{A}{K}, b=\frac{B}{K}, c=\frac{C}{K}, d=\frac{dK}{K} \text{ and } n=\frac{N}{K}.$$
The MathJax code is: $$a=\frac{A}{K}, b=\frac{B}{K}, c=\frac{C}{K}, d=\frac{dK}{K} \text{ and } n=\frac{N}{K}.$$
I see this:
We put $$\overline D=\limsup_{n\to\infty}\frac{w_n}{\sum\limits_{k=1}^n w_k}\text{ and }\underline D=\liminf_{n\to\infty}\frac{w_n}{\sum\limits_{k=1}^n w_k}$$ where $w_k\ge0$ are the given weights.
The formula in this text is: $$\overline D=\limsup_{n\to\infty}\frac{w_n}{\sum\limits_{k=1}^n w_k}\text{ and }\underline D=\liminf_{n\to\infty}\frac{w_n}{\sum\limits_{k=1}^n w_k}$$.
This is how it is displayed for me:
