Underwater Magma Blocks were increased/added in the aquifers to give players a place to breathe. The develops keep increasing the number of Magma Blocks to help achieve this, but if they get buried like this, then their purpose will be defeated. 

Steps to reproduce:

Seed = 4701841352648295399

/execute in minecraft:overworld run tp @s 408.67 27.00 520.68 2035.86 33.30

In the given example, only 1 Magma block is exposed originally upon world generation but if you dig around it, 6-7 more Magma blocks are revealed, which were originally buried.