Ahh! It’s trying to style it as code!
Markdown treats a code block created with ``` the same as an indented piece of text.
As you’re using spaces to centre the headline, it’s interpreting it as a block of code, which is then highlighted, causing the red.
I’ve found the best way to stop it is with the [center] BBCode tag, that way you can centre and bold your headline without any other issues.