The 50 miles from TS is an ice crystal icing problem with the GE engines. They've tweaked the engine software to reduce the altitudes where this is a problem.

I guess Boeing never thought people would fly the -8 empty. If you're very light and very high and going slow (.81-.82) the thrust is so far back the IP bleeds can't provide enough air. "System logic" opens the HP bleeds. But the system wasn't designed to use high pressure and high temperature air in cruise. So the pneumatic system over temps, shuts down and leaves you with no bleed air for pressurization and smoke control.