Very strange.

I've never seen that before,these strange graphs seem like ones on some UIs like anvils etc.

It might also happened in my other saves,but I didn't observe that,and they're gone in a new save.

And they are also visible in the void.

The save is a superflat with a grassblock layer,20 water layers and a bedrock layer.

I hope that's not a feature,that's so horrible!!;(