Weird bug ! Funny you should mention it, because I just had this bug appear in a game I am playing now. The Tile Yields disappeared in three tiles (only 3) of a cluster of five, when my Sov rode across them, from the SouthEast, to the NorthWest, after I fought a battle on one of the tiles. (Strangely, the tile yields only disappeared in the three tiles which included essence. The two tiles which did not include essence were not affected.) The missing Tile Yields did not reappear, when my Sov rode back, from the NW to the SE. I thought I had lost them for good ...
BUT, then after I rode further SE, out of view, and then rode back NW, all of the yields reappeared, when the tiles came back into view !
Weird bug ! ... anyways, as Kongdej says, it is worthwhile to remind the Devs of bugs (even silly bugs) like these ...