After downloading the complete DFS AIP there are all plates in my iPad and i can use it offline. When i do the download again and checking for updates, all the smaller airfilds are not there in offline mode and no longer shown in the list.
What happened?

I can confirm this behaviour.

All plated of the AIP or DFS VFR are downloaded initially and are showing with a green background during the planning. After one goes offline to go flying the plates of the planned route are still available. but plates from other airports cannot be accessed. The problem is that this is random. Sometimes in flight all plates are available, sometimes they aren't. I was CO on a flight yesterday and my iPad had all the plates, while the Pilots iPad didn't show them after take-off. On the ground, being online again, both iPads had all plates available, without having to download them again.

This is quite a severe bug. I'd like to be on the safe side in case I have to land somewhere else due to weather.