I tried a flight in another part of Italy, same problem.
I tried a flight north of Barcelona in Spain, no problem.
The problem is therefore confined to Italy.
Thinking it was a problem with ‘World Update IX’ I removed this, but apart from no longer seeing the castle of Celano the terrain problem remained.
I reinstalled ‘World Update IX’ and disabled the rolling cache, but the terrain problem still remains.
Fly in Italy and check if you have the same problem. Thanks!