PayPal: sudden "Payee(s) passed in transaction does not match expected merchant id."

We have a PayPal integration for a multi-merchant e-commerce site which has worked fine for the past two years. We've been having our members configure integration just by giving us the email address for their PayPal account.

In our checkout we have been loading the SDK with a tag like:

<script src="https://www.paypal.com/sdk/js?client-id=ABC_123_XYZ&merchant-id=seller@example.com&currency=USD&commit=false&locale=en_US&intent=authorize"></script>

where ABC_123_XYZ is our API client id from the PayPal developer portal, seller@example.com is the email address for the seller's (not our!) PayPal account.

And then when setting up the 'purchase_units', we use:

'payee': {'email': 'seller@example.com'        }

And it's worked fine.

Today (March 21 2025) we suddenly started getting the error message:

Error: Payee(s) passed in transaction does not match expected merchant id. Please ensure you are passing merchant-id=XXXXXXXXXXXXX or merchant-id=XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX to the sdk url. https://developer.paypal.com/docs/checkout/reference/customize-sdk/

Fortunately, they're giving us the merchant id in that error message. So after a failure for a member we can start using that ID in the SDK tag, and that seems to work. But making all our members suddenly provide their Merchant ID when they've been able to just use an email address is painful. If anyone has suggestions, or at least an explanation of what's changed that I can pass along, it would be appreciated.