Question on convention of long vowels in poetry

Is it convention that all vowels followed by two consonants (even if in different words) are considered and marked long even if it's naturally short?

If so, who conventioned it? Is it from the ancient times, or is it a medieval or modern convention?

Examples: ānnūs novus / ānnus ōctāvus
(ānnus has short u; octāvus has short o)