Apparently, a change from 1.13 to 1.13.1 was that mobs now require 2 blocks of air to spawn at all times, which pretty much breaks every creeper farm out there.
I've found this thread, but couldn't quite figure out whether it has been recognized as a bug or as a feature: MC-136226
If this is, indeed, a new feature, is Mojang aware that it breaks all creeper farms? Or even spiders, I'm guessing they won't spawn in 1 block tall 3x3 areas either.
Most importantly, does anyone know a way of working around this issue and still manage to make a creeper only farm? I need the gunpowder for my firework rockets...
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It seems as though MC-136226 was marked as fixed for 1.13.1-pre2, so this behavior shouldn't be present in the full release. There are two possible situations this report can fall under:
1) This report is resolved as duplicate and the other report is reopened.
2) This issue is marked as cloning the other issue.
In the meantime, do you have a world that demonstrates this bug? If so please zip and attach it to this ticket (or upload to a file sharing service and link it here). The world save folders are found in [.minecraft/saves|http://hopper.minecraft.net/help/guides/finding-minecraft-data-folder/].
If this was an intentional change, then I don't think it can be classed as a bug. Try suggesting the reversion of this change on r/minecraftsuggestions.