The issue used to be related to the landscape components going nuts.
You know there are several ways around NOT redoing your work from scratch.
The top one I have used several times (this has happened while working on a map more then once with varying degrees of damage to the map itself).
create a new level in a new folder. Save it.
enable world comp, save it again.
copy the level that is broken into the folder.
close/re open the world comp level.
Your level will auto load in the levels tab. Load it in with a right click to visualize it.
Create a new level with matching landscape size inside the same folder.
re-cycle the level to auto load it.
now you are ready to move things over.
Load the level you want to move things from, select everything except the landscape.
go to the levels tab.
Right click the nee level and select the option to “move actors to level”.
Next up copy the level BP over if you have any.
You’ll need to recreate the variables and I have no way around that. However with all the components copied over most things usually just work.
Now you work on the landscape within the new level. You know the process on that already.
export the heightmap, create the new landscape using it, apply the correct material.
then you deal with the layers. Copy all of the shared assets to a different folder, click the landscape layers and instead of creating a new one assign the copied one to it. This should transfer all your layer work / colors.
Once done you should have a working new level. You can move it away from the world comp folder and open it up to find it be identical to how it was.
Now - consider the fact that you may be able to bypass half of this by just doing the landscape portion.
copy the level, delete the landscape. Export the heightmap off the old one, create the new one, apply layers.
9/10 that solves any issues with it.