The getChunkSnapshot() call in CraftChunk is not mapping raw IDs to block ID:meta values properly, preventing dynmap from functioning properly.

To correct this (I'll put in a PR as soon as my CLA is approved), change the line

IBlockData blockData = (IBlockData) net.minecraft.server.Block.getByCombinedId(baseids[j]);

To:

IBlockData blockData = (IBlockData) net.minecraft.server.Block.d.a(baseids[j]);

This works (by testing on Dynmap, which depends upon this call), and is based on the corresponding implementation in ChunkSection.getType(int, int, int).