The information from the BMI on this topic is quite clear –
“If you are the holder of a residence permit “Artikel 50 EUV” with a validity period of 5 years you should apply for an extension before the 5 years expire. You won´t receive a reminder. Therefore, please make a note of this deadline and set a reminder in your calendar. However, a later application does not automatically lead to the loss of your right of residence.”
“Wenn Sie einen Aufenthaltstitel “Artikel 50 EUV” mit einer Gültigkeitsdauer von 5 Jahren haben, sollten Sie vor Ablauf der 5 Jahre um eine Verlängerung ansuchen. Sie bekommen keine Erinnerung. Notieren sie sich daher diese Frist und setzen Sie eine Erinnerung in Ihrem Kalender. Eine spätere Antragstellung führt aber nicht automatisch zum Verlust ihres Aufenthaltsrechts.”
There are only a few reasons for someone to lose their residence rights and that is being absent from Austria for more than 6 months in a 12 month period (12 months for exceptional reasons) or some sort of criminal activity. I am not aware of anyone losing their A50 residence rights yet!
Just in case, here is the appeals process – https://www.britishinaustria.net/the-guide/residence/a50brits/art-50-euv-card-appeals/
and if you don’t get and joy there, you can take it up with the Volksanwaltschaft – https://volksanwaltschaft.gv.at/en/
