Light levels underground are showing 15 sky in all environments. Replacing stone with dirt seems to fix this. Snapshot was upgraded from 18w16a and was working before upgrade.
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I can confirm that I am having the same issue. Please see the attached screenshot of a cave spider spawner room, surrounded by stone, but light levels are high enough to affect spawn rates.
Workaround: Destroy all blocks and replace with dirt.
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Slime farms are working overtime because everything is seen as transparent above (as seen in below screenshot).
Villages are not working too as stone roofs are seen as air and therefore not a valid building.
I have also been upgrading up through the snapshots.
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Am also having this issue after upgrading a world from 18w16a to 18w20b.
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