Are you sure they get deleted? The only thing I can think of is the following scenario: You have more than 100 factory buildings, so some factory buildings share a surface. There may be two factory buildings sharing a surface that both have roboports installed (especially since you're using No Belts). When you pick up such a factory building, power gets cut in that building, and roboports stop receiving power, but the robots inside are still busy and use up power. Once the roboports are completely drained, the still-active robots are forced to migrate to a working logistic network on the same surface, which in this case means they must slowly fly across the void to the other factory interior.
So my guess is that your robots are just flying across the void and will soon arrive in another factory building on that surface (if they haven't already arrived there).
