When using /locate near a biome you have used /fillbiome on, it does not locate that biome, but a naturally generated one. This is easily fixable by changing /fillbiome to detect generated chunks with a certain biome, and not their initial one.
MC-267559
/locate biome will not detect biomes created by /fillbiome
Resolved
Won't Fix
0
Pretty much the same as MC-251311.