May it's an exploid, cause building small ships with much sensors on is more effective than building construkter to upgrade the starbases with sensoremoduls.
You could build a ship with a sensor range of 16 in one round , were it takes much time to build 4-5 construkter for the same effekt.
There is no reason to upgrade the starbase, cause ships are not only much cheaper, they are mobil so you can allways move your Sensorship were ever you like and you could install a small weapon on it to fight the transporter an other unarmed ships of the AI.
I think hat should be fix in one of the next patchs.