Auto-miter works great if you select a face loop. In your case, you have
selected two adjacent edges and (while not intuitive) what you will need to
do will be create a corner seam. For help on how to do this, type in "sheet
metal" on the Index page of the help system and the very first entry is
called: 3 bend corner
Select that and you should see a topic entitled: Add corner seams to sheet
metal faces
Towards the bottom of that page (under the note) there is blue arrow next to
text that reads: Three-bend corners
Select that and you should see the exact case you are trying to create.
All that said - I agree that your method should work and making that
workflow behave as you expect remains on our list of things to do.
--
Gary R. Smith
Autodesk Inventor Team
Portland, OR
2.33GHz 2GB IBM ThinkPad T60p; XP pro SP2
ATI Mobility FireGL V5250 driver: 8.293.1.0