Interesting, I've been messing with a few things related to characters being inserted automatically, so I will give this a look as soon as I can! (Kind of in the shader trenches here, but it looks like you're using this correctly, something is just messing up on my end...) My best guess so far just by looking at it is that it's trying to insert a zero-width space, but that wouldn't show anything at all, or maybe it's a hyphen/soft hyphen that doesn't exist in the font? Anything happen differently if "insertHyphens" is set to false?
Either way, I've got an update I'd like to publish this week and I've done so many changes I can't tell if this was fixed or not, so if you send me an email thru my website with your invoice no., I can get you the latest build of that to see if it has a difference? Otherwise, that will hopefully be published soon and we can recap then!