Unfortunately that is beyond my skill. ill ask one of the devs to take a look at it this week if i can repro it
The ZOC bug can be reproduced fairly easily on my system (and I expect on others) by starting a New Game or by Loading saved games until, on loading up the new map, the ZOC has disappeared. One easy way to see this is to start a New Game in cheat mode, reveal the entire map by Ctrl+U, then switch starting places using Ctrl+N (which causes the whole map to reload) - assuming that the map has wildlands (which start the game showing a ZOC), their ZOC will disappear after a few reloads/start position shuffles.
Once the ZOC bug has been induced this way, the ZOC will not re-appear (Even if a New Game is started or saved game loaded) until you Exit the game and restart.
I've also seen the ZOC bug induced by myself or AI players Settling a new town using a sovereign, but this seems more intermittent.