When I focus on @PA the line “Some thing @Pa” is not included but the untagged line Something C is. The copy and paste action here reveals that the missing line is tagged with @P lowercase ‘a’ but it is displayed as @PA by folding text. Overwriting it to caps A does now include the line in the focus. However I have no idea why the line Something C is also included.
Something C is an ancestor (immediate parent, in this case) of - List G @PA
(Lists at the same level of indentation as a preceding body para are interpreted as children of that body para).
When we click on a tag (let’s say @done, for example), the filter is automatically set to ///@done, which (usefully, I think) includes the ancestors of the tagged lines, as well as the lines themselves.
It sounds as if you are expecting a //@done filter (just the matching lines, without their ancestral context).