Facebook Oauth access_token andpoint does not return "expires" value

I have a strange problem with Facebook Oauth access_token endpoint:

https://graph.facebook.com/oauth/access_token?             
    client_id=APP_ID&
    client_secret=APP_SECRET&
    grant_type=fb_exchange_token&
    fb_exchange_token=EXISTING_ACCESS_TOKEN

The documentation here (in "Scenario 3") said: "If the call is made while there is still a valid long-lived user access_token for that user, the returned user access_token from this second call may be the same or may have changed, but in either case the expiration time will be set to a long expiration time."

If I obtain a valid long-lived user access_token, this endpoint returns the same access token I already have and expires parameter too but only for a short period. So if I repeat the same call to the above andpoint with the same access_token after some time (few hours for example) it returns only the access_token without the expires parameter. ...so I can't establish when my access_token will expires. Note that the access_token returned is still valid and it works perfectly.

Have you experienced the same issue? Do you have any suggestions? Thank you!