Riiight...the devs meant for it to be this way as a game mechanic...
So...you can settle on an inhospitable planet and terraform it, but you can't terraform it after you've made it inhospitable by your own hand!
Tree hugger power go!
No way this, and the mess of other things that don't work right could be a bug!
If you can't terraform anything then it shouldn't appear in your build list.
It makes total sense as is. Terraforming technology is only capable of "fixing" a certain level of "poor" terrain. With each project, you get to select which ONE hex to fix. Thus, at each level of technology, a planet will have a fixed "quality" (i.e. total number of usable hexes).
If, during an invasion, you blast a bunch of hexes so badly that your current tech can't fix them, you've PERMANENTLY reduced the total quality of the planet. Adding new terraforming techs can fix new parts of the planet, but the ones you've destroyed aren't coming back.
Really, this is absolutely no different than under GC2, where certain invasion tactics likewise reduced the PQ. What's changed is that each terraforming tech now is yields only 1 hex from a bunch of choices.
It's in no way a bug. Nor is it broken, in any sense of the word.