Can't save level that has landscape layer material when moving level asset.
It look like regression,because it can save level asset in 4.19.
This issue occur also that moving "Landsacape" level asset with "ContentExample" projecat.
At that time, displayed "can't save .umap" message. (attached file)
If it hide the dialog and restart the editor, the .umap file is lost.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-64469 in the post.