Not sure the cause, but if I create a new Flash Builder project/application, add a map instance and go to Design View, it works.
The problem must lie somewhere in my current project which is upgraded/imported from an api 1.3/sdk 3.5 project. I am using 2.1 api and 4.1 sdk so I'm not sure what details under the hood are borked.
It definitely only occurs in components that contain a Map instance. Odd.
I'll try to look into it as time permits.