Per Hank:

for our immediate problem, i don’t have a specific theory of causation.
there are multiple possible causes, and we don’t have enough information
to distinguish among them. i would like to try the suggested procedure
(writing to a tmp dir, testing there, then moving the file into the item
dir, even if that entails a copy across filesystems) because

(1) it has diagnostic value, helping us narrow down possible causes based on
    whether the problem recurs,
(2) if at all possible, i want to preserve the practice of moving or
    copying only complete, known-good, files into the item dir, breaking
    that convention as a last resort once we’ve determined it’s the only way
    to resolve this problem.