The -R prefix marker installed in synced_dirs was discarded when finalizing
the per-directory delete sender (--delete-during/--delete-delay), so the
up-front root plan was the receive root '.', whose keep list only held the
first prefix component. The receiver then deleted destination content
outside the transferred prefix (e.g. unrelated/keep.txt), a data-loss bug;
rsync keeps it.
Confine the walk to the -R prefix: send that prefix's plan as the root plan,
only transmit plans at or below it, and never emit the receive root plan for
a scoped run. Add a differential test covering both --delete-during and
--delete-delay.
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