The same happened with me. I always migrate levels and I never encountered any problems but there was that level and it did the same. Apparently, inside the new project you have to make sure (preferably before migrating the level) that ALL of the plugins used in the old project are there.
In my case it was specifically the “Water” plugin (The new UE 4.26 plugin). When I enabled it, the level showed in my content browser after restarting the engine.