From @jonathanstowe

Consider the following​:

role Foo {
  has $!attribute;
  multi submethod BUILD(Str :$!attribute!) {
  say "with attribute";
  }
}

class Bar does Foo {
  multi submethod BUILD(Str :$foo!) {
  say "with foo";
  $!attribute = "default";
  }

  method foo() {
  say $!attribute;
  }
}

class Zub is Bar {
}

Bar.new(attribute => "set").foo;
Bar.new(foo => "something").foo;

Zub.new(attribute => "set").foo;
Zub.new(foo => "something").foo;

Resulting in​:

with attribute
set
with foo
default
Cannot call BUILD(Zub​: Str); none of these signatures match​:
  (Bar $​: Str :$foo!, *%_)
  (Bar $​: Str :attribute($!attribute)!, *%_)
  in block <unit> at tt line 25

It appears that it is trying to call the BUILD with a *positional* in the sub-class, however supplying the candidate that it reports it is trying to call doesn't effect the outcome, which is somewhat odd in itself.

Unfortunately the call path which leads to this state of affairs is not clear to me at all.