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).

1. Download the attached World_MC-257648.zip file and extract it in your saves folder
2. Run the following command

   /fillbiome -12 -4 0 -12 -4 4 minecraft:badlands

3. Observe that the biome seems to have changed correctly client side
4. Place some blocks around the area and notice that sometimes the biome colors revert back to the previous biome
5. If this didn't work, try running the same command again but with a different biome