Shopify recently started returning an error code for orders tracking when updated via Shipstation

[Paraffle]

I've used Shipstation for my fulfilling my orders for several years now. Recently, Shipstation has been unable to automatically update my orders with tracking. The orders make it through to shipstation, but once I've generated the postage labels (Royal Mail), the tracking information and order status in Shopify doesn't update. 

I've not changed any settings that i can recall that would impact this, either in Shipstation or in Shopify. 

Shipstation's tech support team have investigated, and provided this error message that the Shopify API is returning. They've also said nothing's changed their end that would cause this. So presumably something's changed on the Shopify side...

"2024-02-15 13:27:22,841+0000 | SS.Tracker.TrackDelegate | SS-TRK-H-0a6586 | DelayedNotifications | 55 | ERROR | 9f4e7710-547c-4fd6-b866-05b32b3ee0b3 | Unhandled exception getting tracking information Tracking Service does not know about this tracking number. SellerID: REDACTED, TrackingNumber: REDACTED, ShipmentID: REDACTED, ProviderID: 20, elapsed: 277 SS.Business.Shipping.Exceptions.ShippingProviderException: Tracking Service does not know about this tracking number"

( I've contacted shopify support, but they've said I need to post here.  I've redacted some of the info in the error message as it's data I don't think should be shared publicly on a forum...)