I've been doing online updates since beta14. Today I downloaded the full beta16, saved a copy of thes four folders: ah3terr, chconfig, settings, and sights (working on some updates for the current sights). Ran an uninstall, then a full install of beta16. Tried to go into offline mode and the game would fail on trying to run NDisles.
When I copied over the four folders I backed up, the game ran my custom terrain fine in offline mode. When I then selected NDisles, the game ran it fine in offline mode.