At the edge of Ashey Shoals biomes, the ashes will be chunk aligned and not actually based on biome edges like so:

I haven't looked at the code but I am guessing the ashes are a feature that is very much like the snow-and-ice feature that vanilla uses. The issue is, since this feature is only added to Ashey Shoals and doesn't check what biome it is in, it causes this strange boundary issue.

There's two main solutions to this.

Solution 1, add the ash feature to all nether biomes but only generate the ash if the spot is in the Ashey Shoals biomes. (places a performance impact on all biomes but best coverage of all solutions). Has to be all biomes because if you only keep it in Ashey Shoals with the biome check as well, some edges will be chunk aligned since that chunk's origin may not be Ashey Shoals but it goes into the Ashey Shoals biome.

Solution 2, add the ash on top of the land in the surfacebuilder while keeping the ash feature with the biome check in the Ashey Shoals. The surfacebuilder ash will match the biome edges perfectly. Then the ash feature will add ash on all the features/structures in the biome (including other mod's stuff) and the biome check will help keep it mostly aligned to the biome edge. Where it doesn't align and causes a chunk boundary, it will only be localized to the ash on top of features/structures which is not going to be seen very often and thus, helps hide the issue. (This keeps the performance impact only within Ash Shoals biomes but does not have perfect coverage)