Bug introduced 8.0 or earlier and persisting through 10.4.1 [CASE:3631078]

In Mathematica 8 and 9, defining this rule:

rule = 
 func[a | b, c_][PatternSequence[x_, z_] | PatternSequence[{x_}, z_]] :> {c, x, z}

leads to Pattern::patvar error messages. But it seems to apply correctly:

func[a, 2][1, 3] /. rule
(*  {2, 1, 3}  *)

In Mathematica 10, defining the rule works without error messages, but applying the rule gives Pattern::patvar error messages, although it returns the correct result.

I think I'm doing something wrong in making this pattern. Question: What is the correct way to construct it without getting error messages?

By the way, changing the c_ to a _ in rule makes the error go away:

rule = 
 func[a | b, _][PatternSequence[x_, z_] | PatternSequence[{x_}, z_]] :> {c, x, z}

But I need the c in the LHS because it appears in the RHS.

By the way #2, if I use this pattern:

rule = func[var : PatternSequence[a | b, c_]][
   PatternSequence[x_, z_] | PatternSequence[{x_}, z_]] :> {c, x, z}

General::mbox errors are generated. In Mathematica 10, the error messages reveal that c_ is getting internally converted to Pattern[1,_]

HELP!

EDIT

I just realized that I can define the rule like this without problems:

rule = func[a | b, c_][{x_}|x_, z_] :> {c, x, z}

But this is an unfortunate case of stripping down too much from my real case which has variable number of arguments on each side of the Alternatives, like this:

rule = 
  func[a | b, c_][PatternSequence[x_, z_] | PatternSequence[{x_, z_}]] :> {c, x, z}

This still leads to Pattern::patvar error messages.