SUMMARY
If 'Recursive Selection' is enabled, all sub albums are included in the Duplicates scan. e.g. The Albums summary says "15 Albums selected". If a sub album is then deselected in the tree, the Albums summary says "14 Albums seleced" as expected.
However, clicking 'Find Duplicates' automatically resets Albums summary to "15 Albums selected"and indeed 15 albums are scanned including the sub album intentionally deselected.

The cause I assume is that the 'Recursive Selection' remains enabled. Manually disabling this after deselecting the sub album gives the expected scan scope. e.g. 14 albums.

The expectation is that if I manually deselect a sub-album, then 'Recursive Selection' should be implicitly disabled.