This happened twice in a row on one night in Realms.

The center of a fenced in potato crop was well lit on the sides but a little dark in the center at night. All the soil was tilled and every potato was fully grown, yet despite this a creeper spawned in the center of the farmland. A friend and I slayed it, but shortly after, I saw another creeper quite literally appear at the center of the crop again.

To stop the spawning, I lit up the center of the crop, but I found it strange the creepers spawned where they did because my friend told me after the first creeper appeared that mobs are not supposed to spawn on tilled ground and later I read elsewhere online the same thing.

I did check to see if there was an untilled spot of land among the potatoes after the two creepers were slayed but there was none.