According to the product documentation, the CA Mobile Authenticator App Expiration value is based on a time-step configured value in the Strong Authentication Mobile OTP profile. The timeout period is equal to 5*(time-step).
App timeout does not work only in iOS CA Mobile Authenticator app.
Symantec Strong Authentication 9.1.x
CA Mobile Authenticator App - iOS
The Symantec Advanced Authentication product team has addressed the issue in the latest version of the iOS CA Mobile Authenticator app. We recommend upgrading the app to resolve the issue.