The TransformedWorldCFrame property of bones does not have the “Read parallel” tag and thus cannot be examined in a parallel context. I see no reason why this is not possible since all the requisite properties like WorldCFrame, Transform and even the similar TransformedCFrame are all read parallel.
This requires annoying workarounds like manually stepping through the transform tree or accessing the value serially before launching the parallel thread. Both cases are not just organizationally unappealing they are also just less efficient.
This has been mentioned previously (TransformedWorldCFrame should be Parallel Safe) but has yet to be resolved.