Yes the appliedtotransaction can return item receipts as well so you'd want to specify the applied to transaction type in your criteria to only get Purchase orders.

So something like this:

const poSearch = search.create({
                type: search.Type.TRANSACTION,
                filters: [
                    {
                        name: "internalid",
                        operator: <http://search.Operator.IS|search.Operator.IS>,
                        values: [searchResult.values["internalid"]],
                    },
                    {
                        name: "type",
                        join: "appliedtotransaction",
                        operator: <http://search.Operator.IS|search.Operator.IS>,
                        values: ["PurchOrd"]
                    },
                ],
                columns: [
                    {
                        name: "internalid",
                        join: "appliedtotransaction"
                    },
                    {
                        name: "tranid",
                        join: "appliedtotransaction"
                    },
                    {
                        name: "trandate",
                        join: "appliedtotransaction"
                    },
                    {
                        name: "total",
                        join: "appliedtotransaction"
                    },
                    {
                        name: "currency",
                        join: "appliedtotransaction"
                    },
                    {
                        name: "memo",
                        join: "appliedtotransaction"
                    }
                ],
            });

should work?