Just like the title suggests, I am experiencing a very strange bug where the following happens:

-I hit end turn with everything normal

-New turn loads with no major event changes (no civs die, no spells are cast by me or others that I can see, I am not attacked by anyone, no random events that I can see)

-Suddenly I see the entire map as if it is not only explored but in view as if I have a unit with sight of each and every position on the map.  I am able to click on all units, view all cities, and I even meet the one civ on the far corner of the world from me that I had no business meeting for ages. 

Map is very large, 16 civs.  Ridiculous/Ridiculous.  The bug happens even if I reload and old save from just before and end the turn in question again.  I'll link my before/after save files in a subsequent post once I've tested further.

Does anyone know what could be causing this?  As convenient as it is I'm really not interested in taking advantage of this bug but there's really no way to avoid it it seems.