Magma blocks are usually generated inside aquifers and flooded caves, however I found some magma blocks generating in the wrong place such as inside clay pools in the lush caves, and even sometimes outside any aquifers and flooded caves, like in generating underneath flowing water that isn't any part of the flooded cave.

Clay pool:

Seed: 1235243567
Coords: 1042 -41 -1992

Seed: 1235243567
Coords: 991 -49 -1963

In the middle of a dry cave:

Seed: -1335482488
Coords: -52 -25 -174