Strange graphs appear near the sun

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!!;(