Nice puzzle. I know a bit about the fuel system of normally aspirated 337's (see the Fuel Supply Management link in
www.SkymasterUS.com), but have not seen the diagram for Turbos.
Does the diagram show the excess fuel and vapors going back to the auxiliary tank in use? It seems that way if you only consumed 12-13 gal in one hour. Two things occur to me. First, see in the POH if the USABLE amount is only 13 gal (I know it's far fetched to have 5-6 gal unusable per tank, but it's worth checking). Second, might there have been some debris which floated into the tubing shutting off the flow? It would be interesting to check flow out of this aux tank on next flight.
Ernie