Thank you for taking a look. After your confirmation I was trying to setup a github TC with a fresh plugin and I ended up isolating the problem myself.
It occurs when using the if helper within a component argument (@icon={{if...}}) inside the modal template. Removing this specific usage makes the modal render correctly.
This is what I was trying to do @icon={{if this.isSubmitting "spinner"}}
BTW I love this new Horizon theme.