This sounds like something I had a while back. Here's my post with some pictures. Not sure if this is the same probelm:
https://forum.awareim.com/d/3547>
I found that the list box did not find its default width when placed inside of a separator with no value in the width field for the list attribute. However, if you add an HTML item first, then the list will properly determine its width relative to the separator control that it resides within.
You can always just set the width but that's no fun. Try my trick above and see if that helps.