I believe the main problem there are (double) spaces in latex expressions.

Most latex renderers do ignore extra spaces but in some distributions of latex extra spaces are rendered as unknown characters with [ ? ]

Editor mode on the other hand replaces double spaces with single spaces - if you use copy and paste in text mode double spaces are not replaced with single spaces...

You could as well write that expession as

\frac{{-b\pm\sqrt{{b^2}-4ac}}}{{2a}}

without any spaces to get

$$\frac{{-b\pm\sqrt{{b^2}-4ac}}}{{2a}}$$