graphical glitch

[Gribeauval]

I have experienced a strange thing.

At the beginning of a turn, the color of the map has changed : a green clear hexagon has become brown for example.
I have thought that it reflected a change of ground conditions (mud appearing).

The bottom of the map was duplicated, there were now two Munich cities on the map for example.
The part of the map that was duplicated has kept its green colors.

The coordinates of the units were not correct, because the whole hex grid and the terrain seem having moved (an immobile unit that should have been in Donnauworth was now 2-3 hex below this town).

I have saved the game.

I have reopened a day later the saved scenario and everything was correct : colors, map, hex grid, coordinates,....