Most maps have borked biomes
I noticed tonight that on Precession at the snow area towards spawn, there was some Mushroom Island biomes randomly mixed in with the normal ice planes. Me, Robin, and Falcon did some more research on this and found several things:
-The problem exists on most maps. Here’s the few maps we found biome errors on:
Precession Abandoned Kingdom Arcane Realms Modern Woolfare
-The problem exists for all versions of minecraft (we tested 1.7, 1.8, and 1.10)
-The problem exists for single player and multiplayer
Our hypothesis about why this happens is: If you have areas of a map where the biomes intentionally change (for example, spawn on Precession is Mushroom Island, the first part of the map is Ice Plains), and you have updated the map file at some point (for example, updating the map from 1.7 to 1.8) then Minecraft will smear the border of your biomes destroying the hard biome border you originally made.
We also found on a few other maps that the biomes wern’t screwed up, including Fourth Dawn, Elemental, and Song of the South. With all these maps, they have a constant plains biome throughout, including the void lanes. So according to our hypothesis, there was no biomes borders for Minecraft to smear, since it was all the same biome.
So the significance of all this is if you have a map where the biomes intentionally change, and updated the map to a newer version at least once, then the biomes will be screwed up. This includes a common case on maps where the left/right lanes have defined biomes, but the void lanes to either side were left untouched by the map maker, and thus have whatever random biomes from the original seed. These random biomes will “bleed” into the lanes from Minecraft’s smearing. In most cases this isn’t a big deal if there’s smearing between an Ocean and Plains biome for example, but if you used Mushroom Island on your map, there’s probably an issue.
tl;dr: Mapmakers, if you know your map has varying biomes (or you didn’t specify the biomes of the void lanes) check your biomes and go back and re-apply the correct biomes if they are borked, especially if you have any mushroom islands, sky biomes, nether biomes, etc in your map.
Update: I just looked at False Respite, which goes from Beach in the main lanes to Mushroom Island in the void lanes to End biome in the iso lanes, and there’s no problems. This is probably because the map was never updated between versions, which confirms the other part of our hypothesis.