Where the first vowel of laciniate went
schwaOnly the SIN of laciniate is stressed. Everything on either side of it is reduced, and reduced vowels get written however they land.
heard asluciniate
spelledlaciniate
saidluh-SIN-ee-ut
It is an a: laciniate.
That first syllable gets no stress at all, and unstressed English vowels all collapse towards the same neutral uh. Written out honestly it would be a u, which is what this rendering does. The a is there for a reason that has nothing to do with sound: the Latin noun is lacinia.
More of the same word
lacciniate laciniat leciniate lasiniate lacineate laciniute
Every one of them has an entry. All 9 spellings of laciniate.