Although the previous two referendums (November 2012 and June 2017) also had ostensibly pro-statehood outcomes, The New York Times described them as “marred, with ballot language phrased to favor the party in office”.
For example, the fourth referendum, held in November 2012, asked voters (1) whether they wanted to maintain the current political status of Puerto Rico and, if not, (2) which alternative status they prefer. Of the fifty-four percent (54.0%) who voted “No” on maintaining the status quo, 61.11% chose statehood, 33.34% chose free association, and 5.55% chose independence.
Didn’t they recently vote to not become a state?
Not as such, but it wasn’t clear cut either. Partly due to the ballot language, it says.
https://en.wikipedia.org/wiki/Puerto_Rico_statehood_movement