tl;dr ChunkMap encodes global state (whether a chunk has been allocated or not) but is used as if it's a local metadata which can cause issues when using multiple spaces that access it such as ImmixSpace and MarkSweepSpace.

For more context see here: https://mmtk.zulipchat.com/#narrow/stream/262673-mmtk-core/topic/.60ChunkMap.60.20with.20ImmixSpace.20and.20MarkSweepSpace