I think I know why this fails.

When the types are in a union and the first value fails, the result is undef and I think* that is what is passed to the 2nd type. If I create a named coercion that covers both types, things work as intended.

declare_coercion "CoerceXeroType",
    from Str,
    via {
        return +{
            Invoice         => 'ACCREC',
            SupplierInvoice => 'ACCPAY',
            Credit          => 'ACCRECCREDIT',
            SupplierCredit  => 'ACCPAYCREDIT',
        }->{$_};
     }
;
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