Many thanks for reporting this issue to us! And it is really strange that the flat pattern error happens using custom equation method when only the corner seam "I AM A BUG" is applied. Actually these four corner seams are created using the same settings and conditions. With more investigation, when the corner relief shape is changed to others like Round or arc weld in this corner seam, the error will not show. However it should be an issue here, and has been tracked in our system.
Based on the model you provided, this should be a symmetrical shape. Please see the model I attached, and hope it helpful for you.
Well that funny I still get the error I guess you did not check it good enough. If I change the corner seam to anything I still get error and this is not symmetrical shape at all times and I never model in that fashon for this causes more bugs within Inventor. But lets get one bug fixed before I go into the others ok.
Please mark this response as "Accept as Solution" if it answers your question.
James Letcher 2012 Factory Design Suite ( will not load 2013) What happen to my Inventor :-(