It appears that grass blocks are showing up as dirt blocks in the minimap, causing the entire minimap to be an ugly brown. Potential interference from Optifine/Shaders? Otherwise bug with 1.12.2.

try and remove optifine and see if that fixes it.

I made a github account just to say I can reproduce this with just using lb photo realism reload on 1.12.2 and so to protect my map from 1.7.10 I don't use mapwriter currently