From this answer:
Note that the nominative neuter ending -u might have been pronounced either as -ŭ or -ū; it seems that we don't have any clear evidence either way for the quality of the vowel in this context.
This strikes me as very odd—I always thought it was known to be a long -ū. Are we really unsure what the quantity of this vowel was, despite evidence in poetry (and potentially fossilized forms going into Romance)? Or am I misunderstanding the answer here?