I have a simple u-shaped part made with a sheet metal base flange and two edge flanges. When I flatten the part it gives a length of 64.589mm. If I make a part with the exact same profile made from an extruded part converted to sheet metal it gives a flattened length of 79.594mm, which is the correct length.
Looking in the knowledge base, S-017112 describes the same problem (2007) and the answer given is incorrect:
Flat pattern length is calculated by - < All flat portions length>+<bend allowance for each bend>. Do not consider the original length of the flanges since the bend radius portion length is also included in the length mentioned in the flange feature.
This is incorrect because it does not consider the centerline length of the bend radius. The part comes out way too short.
In the attached files is an assembly with two identical parts on top of each other. When they are flattened, they are no longer identical.