Hello I two found ways to rename planets who's name vanished after changing to Legendary planets.

1. Recolonize it. I know it can mess up some progress but it the easiest way.

2. This way requires a little patience and steady hand. Change your world's designation to Factory World. In the name change bar the name change options is located in between the O and R (mainly above the O) , ABOVE the word Factory. So put your mouse over Factory, go between the O and R in FACTORY (Mainly about the O) and move up Slowly, the name change option will appear (its in that general area so take your time).

Not sure if intended but settled on a legendary artificial world and no matter how many nexus districts i place, no building slots open up. The modifier doesn't state that it restricts building slots.

never mind it took 4 nexus districts, bit odd though that i need that many