After having used the changebiome command twice, thus using 4 grass, not my entire island changed. The first change was to a Forest biome. 70% of my islands chunks became that biome. The second change was to a swamp biome. 0% of my island became that biome. Even after logging off and locking my island for several hours, this has not changed.
Iv experiences this problem too whenever I changed my biome my whole island doesn’t change only a bit of it does
I am pretty sure this command works off the basis of a certain area will change over. But if your island isn't that big, I can't be for sure. I did it from my spawn and it worked for the most part fine. A few things to consider. One, has your island had the chunks fully despawn which sometimes they won't if people are active near enough to your island. The command works if the chunks despawn. The other thing is, well, is it possible that the command only works for part of the island and not the whole. I really don't know for sure on that part though, only done it on mine once and it worked from what I know.
I've had this issue before but I fixed it by leaving my island for about 5 minutes because if you don't leave the island for enough for some reason only some chunks change biome.
I have heard this happen before - and heard people mention standing in the area that you're trying to change before leaving to let it reset. I have no idea if this actually works, or has any basis to though.
not sure if it helps, but I changed my biome back to plains, and there was a split down my grass where one side was light green and one side was dark green. I went and stood in the dark green area, payed for a change to plains again and it seemed to sort my problem out, and I only have the 99x99 island