Odd, both the app and website work fine for me.
From a legal standpoint, pretty sure usage has to be updated ~72 hours per acma.
Surely you had been migrated to the new billing system by now e.g account starts with a 62.
No I don't get alert emails and even if I did they would not help. The data usage is extremely slow to update and with the unreliable Optus account portal I never know how much data I have left. Yes I can do as you do and stop 3/4 of the way however 50GB is a hell of lot of data to be leaving.
Usage alerts do go out via sms and email.
sms read like this:
137.57GB of 261.50GB of the shared data allowance has been used. At 261.50GB you will get an extra 1.00GB for $10. Usage resets on 16/01. More info on usage at optus.com.au/alerts