Hello @Bryan-Square ,
I was testing card decline use case with following card information but I am able to do payment with following invalid card as per squareup doc. https://developer.squareup.com/docs/devtools/sandbox/payments
CVV: 911 |Card CVV incorrect|
Postal code: 99999 |Card postal code incorrect|
Expiration date: 01/40 |Card expiration date incorrect|
Card number: 4000000000000002 |Card declined number|
PAN: 4000000000000010 |Card on file auth declined|
Would like to know whether we can test this use case in the sandbox or not ?
Do you have the payment_ids for the successful payments with these values? I just tested and they declined as expected. 
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@Bryan-Square Appreciate for looking this issue. It is resolved in our side as well.
Regards,
Pankaj Mishra
Glad to hear it’s working as expected.