There has been a report super simar to this. However it was closed to be a feature suggestion. Here's my explanation of why this is not a feature, but a bug.

So I found NO IRON at all and decided to mine on Iron level. 15 minutes and no iron I turned on xray and no Iron was in sight other than one 5 vein of Iron. I mined it and could finally make a pickaxe for diamonds (sadly i ended up misclicking and making a hoe) but then I turned my world to creative and xray. I found tons more diamonds then Iron as if they switched places