The bug

The flower gradient in existing terrain updated from 1.17.1 to 1.18 has changed. This means that existing flower farms using bone meal are broken.

How to reproduce

Load up a world in 1.17.1, locate a flower forest biome and bone meal grass blocks to produce flowers. 
Load up the same world in 1.18, bone meal the same grass blocks.

Observed results

In 1.18 different flowers are produced from 1.17.1. For example my flower farm producing lily of the valley in 1.17 now produces ox-eye daisies, and dandelions have been replaced with red poppies. In one area cornflowers have been replaced by ox-eye daisies, in another by white poppies. Basically it looks like the whole gradient has changed.

Expected results

The flower gradient in existing terrain should not change, so that existing flower farms can work as before.