do so today (115 packages emerge).  I normally...

emerge -pv --deep --update world | less

...before updating, to check for booby-traps.  Today, the output on the
backup machine blasted to screen, and did not stop until finished.
Meanwhile, the bottom of the screen shows "lines 1-4/4 (END) ".
Actually, it's 151 lines of output.  Similarly...

emerge -pv --deep --update world > x

...shows only 4 lines in file x, while the output blasted to the screen.

emerge -pv --deep --update world > x 2>&1

...does work, sending everything to file x.  That's how I know there are
151 lines of output in file x and output is going to stderr.  This
happens on only the backup machine, not the main machine.  It's not a
showstopper, and I can work around it.  Having said that, I'd like to do
a proper fix.