After loading into an old world in 1.16.0.2, all the grass we can see has gone to the standard green colour. As this area is almost entirely plains or savannah, I do not know yet if it is all biomes that are effected, but it would seem likely that this is affecting all re-colouring of blocks.
It looks like this may have been due to a slightly odd install. I have now moved this world to a new server on the same version and the issue is no longer present.