I'm well aware that others have had this issue before. I'm also aware that it's normally caused by tiles being reverted to grass before showing that they've reverted. However I re-plowed these squares (by hand and by plow) 5 times just to be sure. There was a group of 5 tiles that absolutely refused to be planted with wheat. I could plant wheat elsewhere (and did so), but not those 5 tiles no matter how much I tried.
Eventually got curious if I could plant those tiles with other seeds, went to fetch some poppy seeds. After starting the planting motion (and aborting) with poppy seeds, the tiles could again be planted with wheat.
No clue why that happened, but there's my report/solution.