See this Comm Center thread by Matthew Craig. The Plan Compiler cannot resolve "same folder" (plan-relative) references for #chain, and for #xxxflats. He has solved it with a logic change, and will send his changes for merging here.
REOPENED This logic is broken for simple plans with relative #chain. Found as part of testing on ACP-1911
His patch doesn't handle nested subfolders. I have fixed it for observing plans, but he seems to want flat plans to be handled relative as well. It would be a breaking change given the documentation.