Hotel booking practices in Europe vary region by region, as well as the brand or affiliation of that particular hotel. As an example, we're likewise waiting on a Vienna hotel to open up their schedule following a Sep 2020 GAS Tour, to take a few extra days in that city before flying home. It looks like October (this year) is when those dates will become available. Conversely, we're leading into that tour with a few days in Bruges, and that B&B opened up their entire 2020 schedule this past February, allowing us to secure the nights we wanted far in advance.
As you're now ten months away from (Mailberger Hof) arrival date, would imagine they have those dates known -- whether they still have availability or not. Call the hotel directly, as opposed to waiting for email response, if staying at the same hotel is important to you. Otherwise there are other Wien hotels where you could arrange those extra nights (now) albeit requiring a move between the two facilities. We've done that in Paris (when tour hotel wasn't available), and it's not really that big of a deal to relocate. In fact, it gave us an excuse to get to know an entirely different neighborhood of boulevards, parks and shops. Good travels!