The only thing I can think of is that when you have done it you have also had a TR failed at the same time, or you are in a classic. I think ( from memory) that F/D 1 is from DC bus one and F/D 2 is from DC bus two, both of these should still be powered through the two powered transfer busses and TR1 and TR 2 even though the busses become electrically isolated from each other when the relay opens at G/S capture.
Keep in mind that I may not be correct , this is just what I think and I have not specifically tried it in the sim. Denti I will no doubt show up soonish and confirm or deny, until then, enjoy your day or night, for me it is sleep time.