Problem/Bug Large Minecraft Map Striated When Exported?

So I was working on a really large project map in World Painter for my friends and I to work on for a Dungeons and Dragons event. However when I went to export the map and it finally downloaded to Minecraft after a painstakingly long time, I went to open the world to see if it would be alright to use and this is what I found...

Can someone please explain what has happened or if there is a way I can fix this... I really hope there is because I dedicated some serious time into this.
Pictures are in attached thingy.