So it sounds like for people able to use the app (maybe limited in part due to the use of TestFairy free tier (from a different vice article)), the app correctly got data into Google Cloud Functions, but a script used to transfer the data from there to the Iowa Democratic Party (not sure what they were using) didn't work correctly (due to a "data formatting error"). Some people were confused by the three six digit numbers needed, a precinct id, PIN, and two-factor code (auth0, according to the other vice article with screenshots).
ProPublica also found there was a MITM vulnerability:
https://www.propublica.org/article/the-iowa-caucuses-app-had...
However, even if the app didn't mess up numbers there seem to be a bunch of issues with the numbers:
https://www.nytimes.com/2020/02/06/upshot/iowa-caucuses-erro...