I think the problem is that the union type ends up with two different coercions both from "Str". When trying to coerce a string, it tries the first one, it fails, and then it says "welp; I guess I can't coerce from string then!"

The solution would be to not coerce from Str, but coerce from something more specific.

my %invoice_map = (
    Invoice         => 'ACCREC',
    SupplierInvoice => 'ACCPAY',
);

coerce InvoiceType,
    from Enum[ keys %invoice_map ],
    via { return $invoice_map{$_} };
[download]

Now InvoiceType doesn't have a general coercion from Str, but just from the exact values it knows how to coerce.