With the following, a plugin will receive a wanted constraint that includes a fsk flattening skolem.

-- "Reduced" via the plugin
type family F u v where {}
type family G a b where {}

data D p where
  DC :: (p ~ F x (G () ())) => D p

(Please ignore the apparent ambiguity regarding x; the goal is for the plugin to eliminate any ambiguity.)

A do-nothing plugin that merely logs its inputs gives the following for the ambiguity check on DC.

given
  [G] $d~_a4oh {0}:: (p_a4o2[sk:2] :: *)
                     ~ (p_a4o2[sk:2] :: *) (CDictCan)
  [G] $d~~_a4oi {0}:: (p_a4o2[sk:2] :: *)
                      ~~ (p_a4o2[sk:2] :: *) (CDictCan)
  [G] co_a4od {0}:: (G () () :: *)
                    ~# (fsk_a4oc[fsk:0] :: *) (CFunEqCan)
  [G] co_a4of {0}:: (F x_a4o3[sk:2] fsk_a4oc[fsk:0] :: *)
                    ~# (fsk_a4oe[fsk:0] :: *) (CFunEqCan)
  [G] co_a4og {1}:: (fsk_a4oe[fsk:0] :: *)
                    ~# (p_a4o2[sk:2] :: *) (CTyEqCan)
derived
wanted
  [WD] hole{co_a4or} {3}:: (F x_a4o6[tau:2] fsk_a4oc[fsk:0] :: *)
                           ~# (p_a4o2[sk:2] :: *) (CNonCanonical)
untouchables [fsk_a4oe[fsk:0], fsk_a4oc[fsk:0], x_a4o3[sk:2], p_a4o2[sk:2]]

Note the fsk_a4oc[fsk:0] tyvar in the Wanted constraint, which is why I'm opening this ticket. Its presence contradicts the "The wanteds will be unflattened and zonked" claim from https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker\#Callingpluginsfromthetypechecker section.