Have experienced and fixed something similar a couple of times.
When migrating assets to another project the destination project must have the same
plugins enabled that are used in the levels.
If not the .umap files won’t be loaded or show in the content browser and UE4 will report unrecognized file extension .umap and uasset if you try to import/dragdrop etc.
Sorry for the necro response, just had to “rediscover” this again. 