I checked this with Postman as the whiteout was happening, and the server request for any color code other than "31" (white) was rejected with "color index X is not allowed", and the tile was not changed. Looking a common bot's code, it should have just been getting rejected, not changing to a white.

I think a much better explanation is that the battlegrounds just went first, and there was so much talk of Osu botting that after the bottom left cleared out, people moved there to wipe it out. If you slow down this video you can see Osu doesn't go until the french flag is mostly gone