Hello Robert ,
If I understood correctly, you are trying to convert a surface with fillets into a solid (by providing the thickness). But when that is done, SC does not correctly convert it into sheet metal solid, and hence can not flatten it (Gives an error, "Sheet metal conversion error: Adjacent face is wall face"). Let me know if I understood it correctly.
Trial 1:
I tried the model you shared, recreated the same issue. Although when I tried identifying the form/feature 
, it identified it as form instead of bend (refer to the image below). This identification is incorrect, and I am not clear about the reason. I will have to investigate it further to understand it.
Trial 2:
I tried recreating this issue with a different model (please find attached video), where I did not face any issue. The only difference I can see is the fact that the model you started with itself has a curvature (R5.6m). Although I am not sure if that is the reason behind the false behavior, this will be my primary observation which I will be highlighting to the concerned people.
Regards.
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