Updating world to 1.18 doesn’t replace all bedrock with deepslate

When I opened my survival world in 1.18, I found that some of the bedrock at the bottom of the world had not been replaced by deepslate. The survival world was created multiple versions ago.

 

Expected behaviour:

All bedrock at the former bottom of the world is replaced with deepslate

 

Actual behaviour:

Some bedrock blocks which generated around y=3-6 aren’t replaced

Device info:
iPad 9th generation
iPadOS 15