I am having a similar issue and I know that it is not due to engine version.
I am trying to migrate a level and its associated assets to a new project to trim down file size. The current project is in 4.21.2 and the map loads fine in the current project. I migrate the level to the new blank project using the same engine version and the map is not visible in the content browser (but it is listed in the file explorer). If I try to drag the umap file into the content browser I get: “Failed to import: (directory location) Unknown extension ‘umap’”