Why does this single test case fail, while others work as expected?

Parser.scala

// ...

lazy val prefix: Parser[Prefix] =
  (host | nick) ~ opt('!' ~> user) ~ opt('@' ~> host) ^^ {
    case t ~ u ~ s => Prefix(t, u, s)
  }
  
lazy val host = """[a-zA-Z0-9.:\-^_\-\[\]\\/`]+""".r
lazy val nick = """(\p{L}|[0-9]|[-_\[\]\\`^\{\}\|])+""".r
lazy val user = """[^(\s|@)]+""".r
  
// ...

ParserTest.scala

// ...

"be able to parse nicks with vertical bars (they are in use even if they are not allowed by RFC1459 or RFC2813)" in {
  val prefix = "Miho|[email protected]"
  val successfullyParsed = Parser.parseAll(Parser.prefix, prefix) match {
    case Parser.Success(Prefix("Miho|Zzz", Some("~Miho"), Some("191.ip-149-202-42.eu")), _) =>
      true
    case x =>
      false
  }
  successfullyParsed must beTrue
}

// ...