Why does N-phenylhydroxylamine give Fehling's test?
Quoting from "REDUCTION OF AROMATIC NITRO COMPOUNDS TO N-PHENYLHYDROXYLAMINES - A Comparative Study of the Yields Obtained when
Aromatic Nitro Compounds are Reduced, under Similar Conditions, to N-Phenylhydroxyiaminee by William Henry James"


  N-Phenylhydroxylamine is unstable. It is readily affected by light,
  especially white light. It is also affected by moist air, being
  readily oxidized to nitrosobenzene. It has strong reducing power,
  reducing almost quantitatively Fehling's Reagent to cuprous oxide, and
  causing the formation of a silver mirror when warmed with an
  aramoniacal solution of silver oxide.


So, I guess that its instability and its reducing power are the cause of the effect explained.

Reference


James, William Henry. Reduction of aromatic nitro compounds to N-phenylhydroxylamines. Thesis (M.A.), Boston University, 1932. Boston University Libraries OpenBU