Hello ,
Does this only happen with .umap files and only when using the Save button on the toolbar above the viewport (in the default layout)? If so, this save button is only for the current level and is a way of forcing a save, no matter if changes have been made or not. If you wish to save without saving the level, it would be best to use the other options.
The same situation can occur when using the Save button inside of the Blueprint editor. Clicking this button is a way of telling the editor that you’d like to save this asset, despite if there are any changes.
Also, what kind of source control are you using? When using perforce, if I don’t have the .umap checked out, it’ll ask me if I wish to either check the .umap out or make it writable when I go to save it instead of automatically checking it out as it seems to be doing in your situation.