This is actually not that much kirby related, but perhaps someone can help.
When I use the vertical slash symbol | in a YAML file in sublime text (3) I always get the text below that symbol highlighted in red until the bottom of the document, like this:
I am also using .editorconfig file and plugin on my project’s root so all files have any possible spaces turned into tabs on save, for collaboration purposes.
You could try looking at the syntax theme for the editor and look for that color (if you something like Pigments installed it should be easy to spot). Its just css if i remember correctly. The name for that rule for that color might give you a clue for why its getting that color looks erroneous to me. What happens if you switch to another theme? Perhaps its a bug with that particular one. I use Predawn and never seen it do this.
Oddly it doesnt happen with a clean install of Sublime and onlye Predawn installed, so as @texnixe says, its probably one of the custom settings you have causing it.
It does certainly seems to be the use of tabs instead of spaces what’s causing it, you perhaps can easily try that in this fresh installation, but it seems to be it. yes