When I dug a pit and placed a magma block in the pit, and then placed the water above the magma block and collected it again in a bucket, there would be a pool of water on top of the magma block.
I can't find the relevant content on the wiki, so I think this is a bug and submitted this issue.
Thank you for your report!
We're actually already tracking this issue at MCPE-36463, so we will resolve and link this ticket as a duplicate.
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