According to the Army Air Forces in WW2 Combat Chronology, The only raid on Sofia in Jan 44 took place on the 10th. 15th AF B-17s and B-24s hit marshalling yards and two aircraft were lost. There was one other mission to Bulgaria on the 4th but no aircraft were lost. Seems strange that such a significant loss ratio has gone unrecorded. Maybe unsubstantiated rumour on the part of the author?