Warning with no obvious reason

[MarcMazas]

In the following small grammar, we have a compile warning for which I see no reason.
It looks to me it is a bug (well, I found this while building a very big set of test cases for JavaCC)..
Any idea? TIA. Marc

TOKEN :
{
  < a : "a" >
| < bB : "b" ("B")* >
}

int one_line() : {}
{
  p1() p2() p3() ";" { return 0; }
| ";"  { return 1; }
}

void p1() : {}
{ < ID1 : < bB > < a > > { System.out.println("got ID1"); } }

// here we have a JJ compile warning: ... Regular Expression choice : bB can never be matched as : ID2
// Why? We don't have it in p4() with the direct formulation nor in p3() when not single in the choice
void p2() : {}
{ < ID2 : < bB > | "c" > { System.out.println("got ID2"); } }

void p3() : {}
{ < ID3 : < bB > "!" | "c" > { System.out.println("got ID3"); } }

void p4() : {}
{ < ID4 : "b" ("B")* | "c" > { System.out.println("got ID4"); } }

Following is the remaining of the grammar, if someone wants to test.
Running the test cases shows that the parser does not accept the input corresponding to the warning.

options
{
  static = true;
  UNICODE_INPUT = true; // (default false)

}

PARSER_BEGIN(QuickTests)

public class QuickTests
{
  public static void main(String args []) throws ParseException
  {
    QuickTests parser = new QuickTests(System.in);
    boolean done = false;
    while (!done)
    {
      System.out.println("Reading from standard input...");
      System.out.println("Ex1: babb!; / Ex2: babc; / Ex3: bacb!; / Ex4: bacc;");
      try
      {
        switch (QuickTests.one_line())
        {
          case 0 : 
          System.out.println("OK.");
          break;
          case 1 : 
          System.out.println("Goodbye.");
          done = true;
          break;
          default : 
          break;
        }
      }
      catch (Exception e)
      {
        System.out.println("NOK.");
        System.out.println(e.getMessage());
        QuickTests.ReInit(System.in);
      }
      catch (Error e)
      {
        System.out.println("Oops.");
        System.out.println(e.getMessage());
        break;
      }
    }
  }
}

PARSER_END(QuickTests)

SKIP :
{
  " "
| "\t"
| "\n"
| "\r"
}