One thing that is important to understand is how the two panes in Picard behave. The left pane is for your files that are not matched to a MusicBrainz release. You can change tags there manually and also save the files, but Picard will only ever deal with the tags that are already present in your files (or that you have manually set). So when you clustere the files all the files that had no album information set were clustered into the “Unknown” cluster. Once you manually edited the tags and added artist and album and saved it, these tags where written to the file. Once you loaded it again and clustered again the result now of course used the saved tags, hence you know no longer get an unknown cluster but the album name you set.
What you actually want to do is getting your files tagged with MusicBrainz data. For that you have to get the appropriate releases loaded into the right tag and then assign the files to these tags. There are multiple ways to achieve this:
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Using Cluster + Lookup: This is the first thing you should always try. If you already have your files somewhat reasonable tagged and you mostly deal with entire albums this usually gives the best results. It will search MusicBrainz for a release matching your cluster, load it to the right pane and move the files over.
Note that this relies on existing metadata. If your existing tags are totally useless it won’t be able to find much. Also you can use lookup without clustering, but this will then search on a file by file basis and hence is more likely to scatter your files across different releases.
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Using Scan: This will doe acoustic fingerprinting using AcoustId. This is a good option if you have no clue about the files. As this uses the actual aduio it is pretty good at finding the right recordings, but since this operates on a file per file basis it is rather bad at keeping the recordings together for a single album.
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Loading the album manually from MusicBrainz and dragging your files over from the left pane to the right.
See also the following answer for more details:
Once the files are matched to MusicBrainz data to the right you usually want to look over the result. Usually you can quite just save the ones with a golden disc, but the other wants you might take a quick look at before saving.