I was downgraded, although I already bought capsules this year. Is this a mistake?

Decisive for the maintenance of your status is that you have ordered at least once or the required number of capsules within your membership year (see FAQ “How can I reach the next higher status?”). An example for clarification:

You placed your first Nespresso capsule order on 15 October 2011 and ordered regularly at least once a year. For this reason, you automatically received the status Expert at the beginning of Nespresso & You in July 2018. Your personal membership year is every year in the period from 01 October to 30 September.

So, if you have not placed a capsule order in the period from 01. October 2017 to 30 September 2018, you will lose the status Expert and start again as a Connoisseur with your next capsule order.