After I finished the prologue, up to Vizima. I decided to return to White Orchard, simply because that's the area I've mostly have explored and with only available armour/weaponsmith that I currently have access. Then when I looked on the map, the whole entire field that which I have explored has been reset, so, yet again, I have to explore White Orchard once more.

Is this a bug or is it designed that way for like an extra bonus or something?

seems to be a bug for you sadly, i just went back and checked since i've just left Vizima and its all still visible for me

I've been back to White Orchard numerous times, and it's never reset.

I confirm the above, just like you did, Trench1, I went back to White Orchard after Wizima, and everything was how I left it.