The bug:

Water rivers / lakes generate on top of other biomes with mixed bottom blocks

How to reproduce:

• Generate a new world (preferably with the following seed: 6159632887256740255)
• Go to the following coordinates - X: 150 / Z: 250 or X: -400 / Z: 100
• There are water rivers and lakes generated on top of other biomes (e.g. Forest, Roofed Forest, Birch Forest, Plains)

A small area where 'River' biome is properly generated: X: 540 / Z: -180