Originally posted by Asmosis:

It's an upcoming update, same as ashlands. basically if you're planning on playing the same world forever, dont venture too far towards the edges of the map. Area's you've explored may not generate some/all of the new content when the update goes live, and only the unexplored areas will update.

to add to this
Last I heard the terrain generation was 500 meters past the edge of the area reveled in the map so even if your careful you may end "exploring" more of hte map than inteded.

If keeping your original map is really important to you there is a mod available that will let you reset areas of your existing map so newer terrain generation code can update the area. Using this mod will erase any changes or structures you have made in the rest areas will be lost.

Be sure to back up your world before you use it.

https://www.nexusmods.com/valheim/mods/1485

https://valheim.thunderstore.io/package/JereKuusela/Upgrade_World/