Yes, I remember having read about this exploit elsewhere some time ago. Unfortunately, it has been forgotten here - up to now .
This is a really bad exploit, and should definitely be fixed in the Last patch!
When you build a sphere world, the starbases you used to construct the world with are still included in the "number in service" statistic in the ship design window, even though they disappear after building the sphere world.
I haven't tried this with a ring world, but it probably happens for those too.