Headlights dim

Around 1000 rpm, my headlights (most noticeable)
dash lights and interior lights dim. Always when
braking or decelerating. And only for maybe a
second or two but it gets very dim. Sometimes the
radio pops out for a split second as well but goes
back to normal. I just had a new alternator put in
and have driven on it quite a bit to try to get it
“broken in” per say. But nothing has worked. The
tests on the old alternator came back bad so that’s
why I got a new one. This still happens on the
replacement. I’m not super knowledgeable on cars
but would love to know what’s going on. Thanks!