Normal
Almost every issue with shutdowns under load when using a combination of a AC-DC brick with a DC-DC PSU turn out to the the AC-DC brick being either faulty or underspecced (little/no tolerance for momentary power spikes). AC-DC bricks designed for laptops are not designed to deal with spikes above their maximum rated load (sensitive overcurrent protection) or for rapid changes in power draw (components throttling up and down to reduce average power).