Some props appear solid in creator while driving around, but when testing or playing the track in a race, the props become dynamic.
When trying to switch a different prop into the dynamic category, it will place the prop you chose, but it did not count as a dynamic prop, it counted as a solid prop. It also places the first prop it showed you, but this is not visible until testing or playing the race. This happens I've noticed when you have maxed out your regular props (50 or 100). The worst part of this is it will PICK UP AND REMOVE other props on the track.
I fought with this for a half hour trying to get light generators up in the dynamic category, but it kept putting down runway signs and the light generator as a solid prop. Only because I happened to move around to a different area of the track did I notice all of the barriers I had placed on a corner were now missing except 2.