It sounds picky, but "was finished" has 2 past tense actions. This prompt only has one. There is a meaning change with "is finished" and "was finished." I think they are trying to get us to translate as closely to the original prompt as possible, so we are prepared when the prompts get more complex. (I have no idea what a double past tense action looks like in 'Olelo Hawai'i.)
I get the impression that "ua pa'a" as a phrase can mean finished (?), but plain old "pa'a" is translated as "finished" by itself:
finished — Pukui-Elbert, Eng to Haw, Pau, paʻa, ʻanakoe. Partly finished, kīhapa, laua, hapapū. Completely finished, pau pono, kūpau.
so it seems that "ua pa'a" could / should (?) mean "was finished"? (If not, how do you say that??)