Obviously this I vary rare in Minecraft. I have found many shipwrecks however I have never come across something like this. No, it isn't a bug. I can confirm that this was part of the aquatic update which was released quite some time ago now. In real life, things like this can happen from tectonic plates moving and the earth's landscape changing which believe it or not, happens all the time. Due to this, there is no surprise that this could in fact occur in your Minecraft world.