Before the Psalm for the day at the end of Shacharis each day, we say a sentence, "Today is the Xth day of the week..." In that sentence is a word for Levites, and that word in Hebrew has a sheva under the lamed. Why is that sheva a sheva na? (All six siddurim I have checked show it as a sheva na: Artscroll, Koren, Metsuda, NeHalel, and the two reported here https://judaism.stackexchange.com/a/82836?noredirect=1.)
What rule is being followed that tells us that it is a sheva na?
Update: Two sefardic siddurim I checked showed this sheva as a sheva nach. Perhaps the sheva na pronunciation is only in ashkenaz siddurim.