Not sure when you were testing, but we do call out some instability on the Fidelity Institution Status page in the Developer Dashboard.
> To maintain system stability, Fidelity currently limits access during high-volume windows. As a result, please expect unavailability between 9-10:30am and 3-4:30pm ET. We recommend end users link Fidelity accounts between 5pm - 9am ET.
> To maintain system stability, Fidelity currently limits access during high-volume windows. As a result, please expect unavailability between 9-10:30am and 3-4:30pm ET. We recommend end users link Fidelity accounts between 5pm - 9am ET.