FoldingText queries are outline aware.
All you have missed is the first segment of the search string, which indicates the outline context to search in.
//@today means nodes at any level of outline indentation which are tagged
/@today is top level nodes flagged
/*/*/@today would be level 3 nodes flagged as
if you skip the prefix, it is interpreted, by default, as a search for top level nodes only, i.e.
@today is assumed to be a shorthand for ‘/@today’, so only the first two nodes (level 1) are matched, and the third node (a level 2 child) is not.
(See FT > Help > Nodepaths Guide > Axes)