If you save the game at the beginning of a turn and reload lots of things are not being memorized by the game.

The most obvious one is the units that were just created that very turn: they are not there when you reload.

Also all sorts of locations/resources may not be there anymore or new ones can appear.

I am not 100% sure but I believe that technologies (and spells?) that have just been learned are suddenly "forgotten"

Not sure about strategic spells that are just being cast,  but I suspect that too.

What about children that are born or grown up that turn?

Maybe also other events and things that are harder to spot, once the cause has been understood hopefully everything will go on place.

Looks like a real bad bug to me...