Hi Daan, I don't know about this one, but will share a similar experience to see if related.
When I define a trigger on one of the standard button states, for example "visited", and I have not physically added that state on the button, it will be highlighted in this way. As soon as I add the state to the button, the highlighting is gone. I could (and do) still use the trigger without defining the button state, but it's letting me know, I haven't added a state to the button, which is useful information for me.
It's telling you something, and I don't think it's broken, but what it's telling you, I'm not sure…sorry.
Hopefully one of the Articulate staff can get on here and let you know.