IPN integration from PayPal to MailChimp not working any more

This was working fine for a few years - and stopped working in August 2018.

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I have confirmed the URL provided from MaiChimp (in PayPal, user.us3.list-manage.com/subscribe/paypal-ipn?u=GUID ). I have removed the configuration, disabled IPN to MailChimp, and set it up again - still not working. I have tried manually sending through a recent December IPN - but it never arrives.

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In MailChimp the Integration configuration says :

Connection status

• Connected and receiving data
• Verified: 8/23/18 6:56AM
• Last message: [user@domain] received, though already subscribed to list

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The MailChimp documentation states :

• Someone made a purchase but wasn't added to my list.
  Purchases made through PayPal Express Checkout use a different system than traditional PayPal purchases. Because of this, we can't receive subscriber information or transactions via the integration when a customer makes a purchase through PayPal Express Checkout.

I don't think it's likely that everyone pre-august was traditional PayPal and every since was PayPal express but I guess it's possible. I don't know how to confrim that for a specificed IPN. I see these but no idea if that means Express or not?

notify_version=3.9&custom=&payer_status=unverified and payment_type=instant

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I'm stumped on how to resolve this issue. Does anyone have any suggestions?