In Positive, only update the kind of variables not in delta.

types/types.ml

@@ -2843,7 +2843,10 @@ module Positive = struct
     apply_subst ~subst:subst t
 
   let substitute_kind delta kind t =
-    let subst d = var (Var.set_kind kind d) in
+    let subst d =
+      if Var.Set.mem d delta then var d else
+        var (Var.set_kind kind d)
+    in
     apply_subst ~subst:subst t
 
   (* We cannot use the variance annotation of variables to simplify them,