When converting a world previously made in 1.16.5 and never updated into 1.17, bedrock was non-existent under the already loaded chunks. However, there was bedrock under the new 21w44a chunks.

No Bedrock.mp4

^{This is a small clip from my stream on the new update showing that there is no bedrock.}

^{I know the bitrate wasn't very good so I took a couple screenshots as well.}