It is definitely not a bug as far as the terraforming mechanism is concerned. The terraform techs can be applied to tiles with a certain minimum amount of land in the tile. The really good techs can work with 0% land. The generation of class 20+ planets seems to result in much fewer tiles of the easier land to water ratios. That, to me, is where the fault lies. Having a class 20+ planet shouldn't preclude minimum and marginal tiles or even really affect their percentages. Margins and coastlines would occur regardless of the actual size of the continents. So, I put this down to a behavior in the planet generation procedures that I consider having unintended consequences on the terraforming process.
The devs seem to think that having a 20+ planet should be considered something to be happy about in the first place, and that wanting full terraforming on top of that is just plain greedy. They may have a point there.
However, it would be nice to have terraform techs that can't be used become greyed out with a hover tip that says "no tiles qualify for this tech" or something like that. It should be a principle that everything the game does or doesn't do. it explains somewhere.