The only thing I can think of is some aerodynamic difference, but if they perform relatively the same then that's unlikely, in the 4B I don't see fuel lines running from the outer nacelles to those at the fuselage, I don't see how that would make a difference in the way you're describing, but it's worth investigation. I'm about to check and see if the the structural pylon switch might have been the issue. I couldnt find any reason that that would have been the issue, even the structural pylon's advertised lack of crossfeed turned out to be only advertising. Such to say it still has crossfeed. I don't suppose there was any change to your ascent profile? I figure not but its worth asking. Flying the rapiers on rocket mode when they were previously on air breathing or vice versa? Etc. beyond that I'm stumped.