I captured a planet of PQ19 and have built the starport on a tile with 100% manufacturing bonus. A few moves later I noticed that, decommissioned the starport and constructed a factory on that tile. But after that I could not build a new starport on that planet - it wasn't on the list!
Seems to be a bug in the code which disabled construction of several starports on a planet (I guess it was in early betas of 1.2).