I just randomly found an empty spawner in my world that was converted to the new terrain generation at y:-47.
I haven't seen it on any other world so far but it may effect old an new worlds.
I don't know witch mob it was supposed be.
Old spawner that I found before the update still work.
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The same thing happened to me, I blended an old world and after that I found a spawner in a dungeon that the game timed to make look like a cave or something. I checked the seed again on a fully 1.18 world instead of a 1.17 converted to 1.18 world, and it was normal on there. So it has to have something to do with blending.

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I have the same problem only this Spawner in a Mesa Biom is totally empty. Since 1.18. I think it is a cave spider Spawner. This is a World from 2019 World seed is - 579189559, the Spawner are in new 1.18.chuncks.