Minimap disappears

Whilst playing the Undergames today and yesterday, some way into the game (after 2 hours the first campaign map, and 40 minutes on the second), the minimap goes entirely blank - just showing Fog of war, despite having been populated normally.

At first I thought this was some fiendish new detail in the campaign, but if I save a game, quit WFTO and reload it - the map returns, so I'm filing it as a bug instead...