Either you didn't opt into the public test beta branch, or you came too close to those areas before and they didn't update.
I've also read that having used map reveal at some point via console commands may cause everything to flag as explored, even if hidden again, causing nothing in the future to update - but I can neither confirm or deny that myself.

In any case, as Suzaku mentioned, you may unfortunately have to explore Mistlands on another world anyway. On my 500h+ launch old world, Mistlands are mostly tiny, sheer untraversable sections which in new map generation are supposed to be just the shores.
The space they left for the biome in old worlds just isn't big enough for it to properly generate most of the time.