Over the past couple of weeks or so, I’ve been unable to access Ambrose Island’s mission and other offline content (Pick Your Adventure in Sapienza and Marrakesh from the main mission menu, but can access PYA on the campaign for Hitman 2016). I do wonder if this has something to do with an update for the Offline Content pack, since this seems to have started since a recent update for it.
Remove the mods. Once it works correctly, slowly add the mods back in one at a time. When it goes missing again, you know which one is causing the issue and can remove that one specifically.
I did various things, including deactivating mods, updating and reinstalling the mod (the Offline Mod Enhanced and Custom Mission pack), reinstalling and updating SMF, and everything short of reinstalling and revalidating the game’s files (which I don’t think is the problem–more on that in a minute), and that hasn’t fixed the issue.
However, I did go online briefly to check, and Ambrose Island shows up there, which makes me think it’s not the game itself. I think that there’s some glitch in the mod when it got updated a few weeks ago. I wonder if anyone else has encountered the issue and what their fixes are, or if the mod itself needs some work or some modification, because until it updated, I never had any issues with it.
A typical issue from the IOI server… 
An email might do the job.
Online, Ambrose Island shows up where it should. Offline, it doesn’t at all.