Have you positioned the sensor at the correct position? That rotten thing termed "back focus".
Apparently for the 2" it is 57mm optimally and has a tolerance of +/-4mm meaning anything from 53 to 61mm. The RF mount has a flange distance of 20mm, meaning you have to find around 37mm more between the flattener and the sensor. How are you accomplishing that - 37 seems a fair amount to accomplish.
Next full frame = 36x24 = 44mm diagonal so 44mm diameter image circle. The flattener seems to avoid mentioning a specific image circle - which is poor. Lets face it the image circle size is 100% relevant to buying a flattener.
In the Review of the flattener it says:
"This one performs well over a 40mm image circle in all three small RCs (6", 8", and 10")."
That seems to be the only mention of an image circle, and is insufficent for a full frame sensor. I will agree it says in RC's but cannot think why the image circle should alter greatly. So to me it reads that the flattener is unable to support a full frame sensor. Or at least a good chance it cannot.
The eample image on the description page says:
"The image shows the corner stars across the entire APS-C sized sensor"
That is not a full frame.
If it supports and is designed for a 44mm image circle I expect it to be stated plainly and up front. It isn't.
Any flattener that covers 44mm will by definition be close on a full frame, really one needs 48 - 50mm image circle. And as ever I don't think such really exists. Manufacturers try to get by with the smallest margin they can.
So what spacers are incorporated between the flattener and the flange of the DSLR?
Do they add up to around 37mm?