Type checker plugin receives Wanteds that are not completely unflattened

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.

Trac metadata

Trac field	Value
Version	8.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	adamgundry
Operating system
Architecture