In rare cases the /fillbiome command can cause the biome colors to alternate between the set and previous biome.
How to reproduce
The behavior shown in the attached video doesn't happen at every fill region, so for convenience I created a world where it works every time (that I tried at least).
Download the attached
file and extract it in your saves folder
Run the following command
/fillbiome -12 -4 0 -12 -4 4 minecraft:badlandsObserve that the biome seems to have changed correctly client side
Place some blocks around the area and notice that sometimes the biome colors revert back to the previous biome
If this didn't work, try running the same command again but with a different biome
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Comments 3
The video looks kinda similar to MC-173723, might be related?
In MC-173723 the colors fix themselves permanently after getting a block update. In this case it can alternate between the old and new biome colors whenever a block update happens.
It should be noted that, at least for me, this can only be seen with Chunk Builder set to Threaded and Biome Blend set to either 3x3 or OFF.