I have this strange bug in my mod and I'm not sure if it's CK, mods, or something simple I haven't set properly.

Firstly I got a complaint from a user a while back that he entered into a cell in my mod and all the NPCs had vanished from then on in that cell. First thought was that maybe it was memory related, but I have since managed to do it myself but it only happens when I do one thing. Normally I just coc into a cell for testing purposes and in order to get quest dialogue going I generate an autosave and then go back to main menu and continue and all is fine, however if go back to the main menu and continue again with the same save - bang all NPC's gone unless I quit completely restart and continue from the same file then things are fine again. The interesting thing is if I don't go back to main menu when reloading the bug doesn't happen.

I have noticed that the NPCs are missing in the save preview.

Any Ideas???