Repro steps:
1. Use a project with HDRP or Universal 7.0.1 or higher
2. Go to the asset store and download "Terrain Tools Sample Asset Pack" from Unity Technologies (it's free) and import all contents to your project
3. Create a material with shader "HDRP/TerrainLit" for HDRP or "Universal Render Pipeline/Terrain/Lit" for Universal
4. Enable height based blend on the material
5. Create a terrain, go into the terrain settings, and set the material to your new material
6. Go to "paint terrain" tools, choose "paint texture" from the menu
7. Add 2 of your existing terrain layers through "edit terrain layers", the layers should be "rock" and "snow"
8. Fill a large portion of the terrain with rock
9. Choose the soft brush, choose the snow layer, and click once on the terrain
10. Go to your terrain material and drag the slider from "0" to "1" and back

Expected result: for the best looking behavior and to maintain parity with 19.2, the terrain should interpolate blending with a fully height based blend at 0, a fully "lerp" based blend at 1, and at any point in the middle, varying degrees of soft height map based blend (when fixing, make sure to do these steps in 19.2 HDRP to see what the correct behavior should be)
Actual result. The blend does a full "lerp" based blend at 0, something almost identical looking at 1, and in the middle, varying degrees of very bad looking blending that don't look anything like expected behavior.
See attached images for expected versus actual result.