Capital captured by cavalry without crossing any provinces between the capital and my enemys' territory

Game ID: 9396575
User ID: 85037551

On day 108 at 4:16AM, the 50th Cavalry Corp (MAINE) captured my capital of Bismarck (MISSOURI) without crossing/capturing any of the provinces between Maine's border and my capital. This shouldn't be possible and yet it happened. This has cost me an enormous amount of resources - not only to rebuild my capital but due to lost morale throughout my territory. What can be done about this?