Do fourth-declension neuter nouns end in -ŭ or -ū?

Question

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?

Answer 1

Kennedy §55 suggests ū with no discussion. But also says only three 4th neuters in common use (genu, cornu, veru). L&S also lists as long.

Aen. 1.320 has it long: nuda genu, nodoque sinus collecta fluentis