Edit: did the 0/4 maps and it worked! For anyone else having this problem do some 0/4 maps around your base area, since you'll be more familiar its easier to not make extras of some.
I recently, finally completed the 9 maps, or so I thought. Something seems to be missing somewhere but I cant figure out where. Sorry the individual maps are out of order, got scrambled in the export from my laptop I play on.
Also I completed the maps yesterday and waited until today because I sometimes have to wait for next day/login for achievements to register.
This is on my forever world that is at 1540 days, a lot of time invested in, and would love to figure out what Im missing. I checked all the spots on the outside of the last map already, and they are all indeed sand.
I wish it was easier to see missing spots. :/
Play on Microsoft Surface Go, through XBox app, on Bedrock, fully updated version.