Try to clear out one of :post-read-conversion and :pre-write-conversion to see which of the two is the culprit. (While you're at it, try to clear out both first just to make sure we're barking up the right tree.)

I just saw one suspicious thing: `ucs-normalize-hfs-nfd-pre-write-conversion' change current buffer but doesn't restore it, whereas the post-read function use save-excursion.

    -- Anders