One of the best new features in R80.20 is updateable objects. You can read more here
I had a customer that asked specifically about using them in NAT rules as they wanted a separate HIDE-NAT for outbound access to public Azure vs their private Azure VN. We discovered that the updateable objects could not be used in NAT rules.
After some digging, we determined that updateable objects when used created a dynamic object on the gateway. You can display them using 'dynamic_objects -uo_show' on the gateway. As you can see below the object has a dynamic object link but not a dynamic object created in smartdashboard, the updateable seems to be a link to this object.
If you use the name from 'dynamic_objects -uo_show' you can create a dynamic object using the same name. This will create a physically link to the dynamic object that was created by the updateable object. You can then use that dynamic object in a NAT rule.
You can see the log below;