Capital City Disappearing

Twice in my game, my capital city just disappeared like the fall of Atlantis.  The first time I thought perhaps I was too far away and just didn't see the roving monster conquer my city.  I just happened to have a pioneer nearby so I sent it back to the original site and rebuilt the city.  I made sure I stayed close by the city until after I had two spearmen built to garrison the city.  Then I went off adventuring again.  Then a few turns later, I looked over and saw the city gone again.  My two stacks of spearmen were standing mutely by.

The second time the city disappeared, I'm fairly certain it wasn't attacked.  I never had any battles for the city at all.   I would think I would have at least been offered a battle to fight in either case.  Surely a city would have at least one militia unit by default that would grab pitch forks and scythes to defend their homes.  Surely this is a bug.

Has anyone else experienced this?