You can’t access metafields that way in Flow yet. Storefront does something special to fetch metafields values by namespace. In Flow, you loop over order.customer.metafields … FYI if you click add variable and choose the metafield value it will show you the loop part.
Another way to do it is using where:
{% assign my_mf = order.customer.metafields | where: “namespace”, “custom” | where: “key”, “pledgedcards” | first %}
{{ my_mf.value }}