Noticed this too. Not particularly upset though - I'd rather mine a couple big rocks (which I would need to get out of the way anyway) than chip away at an ore field; getting an early boost of copper and iron ore this way made the start of the game a little nicer.
Tbh it does open up for some interesting challenges, like no natural ores and only rocks can be mined, but it still feels like a bug to me. like they reasonably could be rocks FROM tiber that hit the planet but id exoect them then to be way rarer and maybe clustered with tiberium itself..
