Not sure if this mechanic is supposed to be a thing in Bedrock, but lava on top of soul soil and underneath blue ice is not generating basalt.
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Some features have not yet been added to the Bedrock betas.
This beta is a work in progress version of the full Nether Update. Some features seen in Java Edition snapshots may not yet be present.
For the latest list of changes and fixes, please visit https://aka.ms/minecraftbeta
Lava has to be between soul soul and blue ice.