Plains cannot really appear near the starting spawn point. I mean they can even be in the same landmass as the starting point, but they'll still be very far from it.

The spawn point always has some meadows forced around it. If the area was supposed to be black forest, it will force a "star" shape on top of it, which can create those odd pockets.
Like, this random map has triangular pockets of black forests between meadows and mountains https://valheim-map.world/?seed=vWsezigBYF&offset=158%2C-182&zoom=0.092&view=1&ver=0.207.20