It is Seasonal flank Alopecia all right. We have been through this a couple of times and the good thing is that it will pass. The bad is it takes time. From a couple of months and upwards. If you have show-plans coming up soon there could be problems..
But, I have seen bulldogs with this even in the "Bulldog of the Year" show in UK so it is fairly common. The reason why they get it is not pinpointed yet. I have seen research that indicated this was overrepresented in the northern parts of the world were the lack of sunlight exposure in the winter was the trigger..Others think it is imbalance in the hormonal system who is the main reason, some think it is food related and some even think it is caused by allergy. Anyway, your dog will be fine, it just takes time