There is always a small risk when transferring in a country where you need a visa to enter. Quite often people get stuck in the transit zone due a missconnection and can't leave the transit area to stay at a hotel outside for one night. That's the reason why some countries require a regular visa even for just transferring ( and I don't mean the Schengen transfer visa).

Anyways, Tom Tulpe explained pretty well want went wrong. Arriving and departure gate were both non-Schengen gates but are separated by the Schengen area. Probably no one at Eurowings thought of this.