Just putting this here in case anyone runs into the same issue and wants some sort of answer, I received this email from AuthNet Dev support.

Hello,

Thank you for contacting Authorize.net Developer Support. I understand you need assistance regarding the country-of-origin code for some of your clients. I am here to help.

If the merchant received an email from Mastercard that they are not compliant, then this is more likely related to Mastercard compliance mandate and not a development issue. It also depends on who the merchant is processing with.

You may confirm with the merchant if they received an email from Mastercard. If so, then the merchant does not have to do anything as Authorize.net is still working with this mandate.

You may visit this link for more details: https://support.authorize.net/knowledgebase/Knowledgearticle/?code=000002704

I hope this information helps. Have a wonderful day!

If the information provided above satisfies your needs, please close this Support Case. Otherwise, please add an update to this Support Case with your follow-up questions so I can further assist you. For your convenience the Authorize.net Support Center, located at: https://support.authorize.net. You may also call us at (877) 447-3938, available 24 hours a day, 7 days a week.

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Regards, Authorize.net Developer Support