Thanks for your reply!
Making sure that your assets get scanned correctly solves a lot of trouble.
Totally agree. But I think that 'unscannable' servers (for whatever reason) should not vanish without a reason.
> Another reason for them to disappear could be a cleanup option that deletes assets not seen in XX days.
> Stale DNS records or bad DNS resolution can also cause similar issues.
Sure. I checked DNS and the computers can be resolved. The cleanup options are default:

So I would not expect the discovered servers to be deleted that quickly. Having said that, we've installed an LsAgent to overcome this issue and that's a workaround with which we can live.
We're currently running 12.2.0.4 and will update to the latest asap.