Replacing bedrock with deepslate in old chunks causes a notable discontinuity

The bug

With the terrain blending feature being implemented in 1.18, old chunks are being converted to fit with new generation by replacing existing bedrock with deepslate. This, however, causes a discontinuity between old and new chunks to arise.

In new chunks, ignoring structures and terrain features, blocks at y=0 will always be solid stone, and the transition to deepslate only initiates at y=-1 and below. Existing chunks have bedrock (and therefore will have deepslate) at y=4, transitioning down to a solid layer at y=0, resulting in the transition to deepslate differing between old and new chunks.

How to reproduce

A zipped 1.16.5 world (1.16.5 is used due to blobs of deepslate not generating, making the transition much clearer) is attached to this ticket, which can be loaded in 21w43a. An area can be cleared out in the boundary between existing and new chunks to see the discontinuity.

Expected behaviour

Either all bedrock would be replaced with stone in existing chunks, and new generation below existing chunks would then use the slow transition to deepslate that new chunks have, or the deepslate transition would be moved up to start at y=4 to match existing chunks' bedrock. Both would soften the boundary sufficiently.

Actual behaviour

The discontinuity detailed above.