Lava pools generated below a certain "y" level where grimstones can generate, do not generate with grimstones, instead, they generate with normal stone.
As you can see in the picture attached, there are caves with grimstone above the lava pool, but the stone where the lava rests isn't grimstone. To the right, you can see many other lava pools with the same issue.
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We're tracking this issue in MC-215844, so this ticket is being resolved and linked as a duplicate.
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