Focus / selected item behavior has changed in Picard 2.7.0b3, for the worse IMO.  Using 2 files from the same album,

Picard 2.6.4 behavior: Click Unclustered Files (2). Click the Cluster button. The focus / selected item moves to Clusters (1).

Picard 2.7.0b3 behavior: Click Unclustered Files (2). Click the Cluster button. The focus / selected item remains on Unclustered Files (0), no longer moving to Clusters (1).