Innan du installerar om appen på telefonen kan du prova rensa cache, Inställningar > appar > Steam > lagring.
Läser mans en på Steam om Mobile Authenticator så står följande.
"Next you'll enter a phone number, one that we can confirm with a text message. If you ever lose access to your Steam account, you can quickly recover it by asking Steam to send a text to this phone number."
Detta tolkar jag som att mobilens telefonnummer är en backup.
Står även följande om man har en ny telefon, vilket borde vara samma som att installera om appen.
Can I transfer my authenticator to my new phone?
Yes, you can.
To transfer your authenticator to a new phone, you'll need access to the phone number associated with your account (so we can send you a text message to confirm it's you.)
On your new phone, install the Steam Mobile App and try to sign in to your account. When the app asks you to confirm your sign in via the app, select "I no longer have access to my authenticator." Follow the steps the app provides to transfer your authenticator to the new device.