Problems with `{{ }}` below `{% %}`
As you can see in the screenshot the first pair of braces is highlighted, but all braces below {% %} aren't:

Currently this is by design. The issue is that there is no way of telling whether {{ and }} are meant to be used with {}-format string or something else. Since double braces have special meaning in {}-format strings they are highlighted.
Starting from the point where we see {%...%}, we can tell that this string is highly unlikely to be a {}-format string or a %-format string. However, anything preceding this will be highlighted normally, that is to say that the potential formatting symbols are shown.
We do have some plans for configurable options for string highlighting, where you could disable the highlighting for formats that you personally don't need. So stay tuned.
In the interim, if this highlighting bothers you, you can actually tweak the color schema to hide this from you by making special characters inside strings the same color as the string.