I found a Magma Cube Spawner in a bastion, wanting to make a Magma Cream farm later I decided to deactivate it by covering the spawner in torches and replacing the top layer of lava with Cobblestone, after doing so I was surprised to find the for some bizarre reason, Magma Cubes were still spawning, so I incased the spawner in a solid cube of Cobblestone Blocks with torches on the center block on each side, and then covering the surrounding area in cobblestone slabs with the exception of the surrounding paths and the island with the treasure. What left me baffled is that Magma Cubes still spawned outside the cobblestone cube. What makes this even stranger is that all the other spawners I have encountered have not done this and have deactivated merely by placing torches on the sides.