Access token not renewed if expired

[klues]

Silent renew of the access token is only done if it expires soon, not if it's already expired. It's possible to manually trigger renewal for expired access tokens by

userManager.events.addAccessTokenExpired(function() {
     userManager.signinSilent();
 })

but I wonder, why it's not done by default? Any specific reason for this, or could it be added by default?