Edit: My question is what caused these corporate customers to churn? Did they maintain spend levels with One Medical while the other customers grew spend? Did they decrease spend below reporting thresholds? Did they stop using (or sponsoring) these services entirely?
Elsewhere in the S-1:
"In 2017, 2018 and the nine months ended September 30, 2019, our top customers accounted for 42%, 37% and 36% of our net revenue, respectively. These customers included Google Inc., which accounted for 10% of our net revenue for 2018 and the nine months ended September 30, 2019."
(Disclaimer: I work at Google, and they do pay for my One Medical membership.)