The pattern seems like a strange formation of stone which repeats itself throughout the map. 

When i go to those locations where the stone things are meant to generate according to the map, i see only regular minecraft terrain with no exposed stone at all.

I have tried it 3 times in multiple snapshots ( same world ( survival )). 

in the first try, i made a map in a version which i honestly don't remember, but it was around 20w12a, and it generated those weird stone things i mentioned. When i did so, i extended the map in a cartography table to the maximum, so when i generated the map then it did not reveal the full map. In a later version i came back to the map and generated other parts i didn't generate back then, and they generated normally. 

the second try was in 20w20b. I generated the map yet again ( though not for the same area ) and it did the weird stone things.

In the third time, i tried in 20w21a and it generated the exact same stone structures. 

I am leaving a screenshot below