[Question] Ruined 5th Mipmap? Is it deliberate?

Hi I’m a little curious as to why some of the normal map in the stock textures have Ruined mipmaps ( 5th level ) .
Is it a deliberate move to that this texture cannot be re-used in project? Because if it’s not, I might be planning to use some of these tileable texture.

It seems to be there on several texture.

Some of the files that have it :
T_Detail_Rocky_N ( 4th mip )

There are more.