Ok, just finished up and seeing your comment. I did notice that for other fields this will be a similar issue, but particularly with this field it's problematic as it causes a rendering issue. (for example background I just left as-is since it does not cause any problems).
To be quite honest, I read through the documentation (perhaps not 100%) and did not find a note about this.
Also, I find the solution of having to turn off twig debug to be problematic and not optimal. You know the module well, but think of a team of developers who always have it on, or someone new that joints a team who always work with it on... it becomes a difficult thing to communicate. I can easily see someone wasting time, not understanding why a box appears for an image when there isn't one.
The timeline should render properly (visually) as to not confuse, even with twig debug IMO.
Excuse the noise, but after further testing -- this solution requires additional work.