Having planted wheat next to a lake and after it had grown fully,i harvested it.The wheat dropped,but only 1 seed dropped with it. I searched around but didnt find any seed. I then thought it couldve been that the wheat hadnt grown fully. I replanted it and used bonemeal to grow it. After i was 100% sure it was fully grown (asfurther usage of bone meal had no effect) i tried again but still no second seed. After that, i tried planting,bone mealing and harvesting it on solid landmass,far away from water, as i thought it couldve been because of the water nearby. unfortunately, i didnt get results out of that either. I dont have any photo or recording to make it easier for you. I know i filled it in as a 1.16 bug but i noticed it on 1.15 as well. Hope this helps