Y=-63 unreachable

In my survival world my sister and i are trying to get the caves and clifts acheivement and i noticed something weird so i made a creative copy and found there was no acces to y-63 like the farthest you can dig down is -62

 

Steps to reproduce

1.create a new world 

2.dig down to bedrock Y=-64and dig in a streight line and try to find some deepslate/ore/other type of stone