This issue usually happens the other way round. Folks change the VRM from the original to a private plate and the emissions details don't follow, at least not in a timely manner.
In such cases TFL say you have to manually register the car with them to avoid penalty charges.
See here.In this case could be that the emissions data eventually updated on the private plate and this was before ULEZ inception in your area.
You've now reverted to the original registration and it seems that the emissions data fields have not yet updated. So I suspect you may need to manually register the car with TFL.
Interestingly the TFL ULEZ checker returns:
"We don't know if your vehicle meets emissions standards
Check your number plate is correct. You'll need to provide us proof if you believe it meets emissions standards – we'll tell you how in the next step.
Results will show charges you need to pay today until you tell us more about this vehicle."Please post up the details page of the V5c for each VRM, redact your name and address and the VIN number and the V5c reference number. Also please confirm the exact model of Range Rover Sport you have. In particular the engine.