I say "Fake" because they are entirely non-hostile, to you. They clip through things, sometimes engage other enemies, and in all ways ignore your presence.
I killed a giant one fake dragon was flying, and it just flew through the ground right after. That was weird. If you can manage to hit them, they'll become hostile, but not in any othe circumstance.

Are these on purpose or a mistake?

Not all dragons are hostile or take notice of your presence. That part is by design, the rest of it I don't think is.

Some dragons are not hostile, or at least not hostile to the PC. It is almost impossible to engage the non hostile ones and they fly off after a while. I have taken to ignoring dragons now unless they fly over my head and make that whooshing sound. That means they have engaged me and I should do something about it.

A script fail to run?

I was running along Withrun and this dragon appeared. It was flying around in a large circle. Stalking. Then it left and went east over the river. Npc panic though. Or I suppose the dragon remind you 'We are here. Are you suppose to do something about us?"

Not all dragons are hostile, however some will simply circle for a while without engaging other creatures in combat.

If you ignore a dragon for long enough, the event ends and the dragon flies off. lol

I have seen Dragons fly over and in circles they didn't land and attack anyone they was just flying around roaring and looking around the guards

then they just flew off into the distance and disappeared.