Why is this so? It goes against the syntax standards that have been outlined, chords contained within brackets. With that in mind, I would expect that the chords would not be highlighted if not contained within brackets.
I found a workaround though (and it's a bit kludgy) - if I put a character (such as a hyphen) and a space before the chord, it won't treat it as such. But still, I think from a standards perspective, choose ONE way of dedicated chord syntax (like your brackets that you've established), and if they're not in brackets, leave them alone — don't attempt to convert them to chord highlighting.
We really should be given the option to choose whether or not the chord highlighting is engaged or not.